The Mishnah explains why it is 25 years:

1) Rebbi Meir said in the name of Raban Shimon ben Gamliel that in the course of 25 years it is possible that the widow may use up all the money that her late husband owes her from the Kesuvah, by doing favors. Rashi writes that this means that she gives bread and salt to the neighbors. According to the estimation of the Sages, 25 years is a long enough amount of time for it to be possible that the entire value of the Kesuvah could be lost through these small presents.

2) The Chachamim, in the Mishnah, disagree with Rebbi Meir. They say that as long as she still lives in her husband's house she can always collect her Kesuvah, but if she goes back to live in her father's house she loses the Kesuvah after 25 years. Rashi writes that, according to the Chachamim, the length of time of 25 years has nothing to do with the favors she does for the neighbors. Rather, the number 25 is significant because that is how long it takes for her to be Mochel, to forego her Kesuvah. If she keeps quiet for such a long time this must mean that she was Mochel her Kesuvah; she does not want to be paid from it, because if she did, she would not have waited so long without saying anything. However, this Sevara -- that she remits the debt -- applies only if she is now living in her father's house, but if she still lives in the husband's house, the fact that she kept quiet for 25 years does not mean she has renounced the debt. The reason she kept quiet is because respect is paid to her in the husband's house, so she is embarrassed to claim the Kesuvah but she has not really been Mochel.

3) I found, bs'd, that the Maharshal (1510-1573, Poland) writes in his commentary "Amudei Shlomo" on the Sefer Mitzvos ha'Gadol (positive Mitzvah #48; in the Machon Yerushalayim edition, this is found on page 828, next to number 230) that the number 25 years represents "a long time, without any reason given either in the Gemara or in the commentaries."

I think that what the Maharshal means is that 25 years is merely an example of a long time. It was never meant to mean that a person is Mochel after exactly 25 years, but rather that Chazal looked for a big number, and chose the number 25 because that is a round and reasonable number, and by that time the widow will have been Mochel.

4) However, what the Maharsha says, that 25 years is merely "a long time without a reason," is rather surprising, since the Mefarshim do usually give reasons for Halachos, so I wondered if there are Mefarshim who disagree with the Maharshal and do offer a reasoning for the 25-year rule.

a) I found in the Nimukei Yosef on the Rif, near the end of the fourth chapter of Kesuvos, on page 19b of the pages of the Rif in the standard editionm DH Mocheles, that he cites our Gemara (104a) and writes that since she stood for 25 years and did not demand the Kesuvah, "Mistama Machlah" -- we may assume that she has renounced payment from the Kesuvah. This is also stated by the Shitah Mekubetzes in our Sugya (DH Masnisin) in the name of Talmidei Rabeinu Yonah, that after 25 years "Mistama Machlah," presumably she remitted the debt. This is also stated by Tosfos to Bava Metzia 112a (near the end of DH Chozer), who writes "d'Mistama Machlah," and by Tosfos to Sanhedrin 6a (DH Tzerichah), who also writes "Mistama Machlah."

b) Our Gemara here, in the first of the widest lines, uses the word "Achilta," which means Mechilah, and Rashi writes in our Sugya (104a, in the Mishnah, DH v'Chachamim), "Machlatah." These other Mefarshim add one word "Mistama" -- that after 25 years we can assume she has renounced the Kesuvah debt. There is a suggestion here that there is some reasoning behind the 25-year idea, that she presumably was Mochel by then, but these Mefarshim are not a challenge to the Maharshal because they give us no inkling what is special about the number 25, so we are still without a clear reason.

c) However, I think that if we look at the Ketzos ha'Choshen 12:1 we will get some idea where the 25-year length comes from, and it seems that this is precisely because he disagrees with the Maharshal. The Ketzos ha'Choshen discusses the Halachah of Mechilah, how one can forego a debt. He cites the Misgeres ha'Shulchan in the name of the Maharshal that Mechilah in one's heart also represents Mechilah. The Maharshal writes that the SM'G proves this from our Sugya, because otherwise how would we know after 25 years she was Mochel since she did not say anything? It must be because she was Mochel in her heart and this works. The Ketzos ha'Choshen strongly disagrees because we have a well-known rule (Kidushun 49b) that "Devarim she'b'Lev Einam Devarim," what one thinks in his heart does not have legal power. The Ketzos ha'Choshen writes that there is a different reason for why our Sugya states that after 25 years the Mechilah is effective. This is because after so many years, what she is thinking is known by the whole world. Everyone knows that if she has not demanded the Kesuvah after such a long time, it must mean that was Mochel. The rule of "Devarim sheb'Lev Einam Devarim" does not not apply when everyone knows what this person is thinking in his heart.

d) So there is a reasoning behind the 25-year rule. Chazal estimated that it takes 25 years before we can be certain that the widow will not change her mind. Chazal knew that before this milestone of "half a Yovel" (half of 50 years), it is not totally certain that she has been Mochel.

5) I would like to add a little bit of explanation to what I wrote above. The Ketzos ha'Choshen presents an unusual picture: where the widow's thoughts, that she has forgiven her Kesuvah, must be known to the whole world. We do not usually require such an extreme certainty in Halachah, but the reason why it is necessary here is because the widow has not said a word; she merely thought that she is Mochel, so it follows that for this to have Halachic significance the whole world must know about it.

a) I think we can compare this to a similar idea that we sometimes find in very different areas of Halachah, the idea that "Yatza ha'Safek mi'Libo" -- there is no doubt in a person's heart any more. A famous example is the question of knowing that nightfall has arrived, so that we can say Shema at night. The Shulchan Aruch (OC 335:1) writes that one must be able to see three small stars in the sky, and if it is a cloudy day then one must wait "Ad she'Yatza ha'Safek mi'Libo," until any doubt has left one's heart and he is absolutely certain that it is nighttime.

b) I argue that, according to the Ketzos ha'Choshen, something similar is required here. It must be known to the entire world that she will no longer claim her Kesuvah. Ten years is not long enough for this. There may be a 90% or 95% chance that if 10 years have passed that she has already forgotten about the Kesuvah, but that is not enough. We require 100%. Chazal decided that we can be 100% sure only after 25 years.

6) I found that the Perishah on the Tur (EH 101:3) also writes that the Gemara here does not explain any reason for the length of time of 25 years, but he says that one can suggest a Siman, a sign or way of remembering the number. This is because the Kesuvah of a widow is 25 Sela'im and every year after the husband dies she foregoes 1 Sela, which comes to 25 Sela'im in 25 years.

(See the Mishnah above on 10b that a widow receives a Kesuvah of 100 Dinar. Since there are 4 Dinar in a Sela (see Rashi to Eruvin 82b, DH Kikar), this means that the Kesuvah of a widow is worth 25 Sela'im.)

However, the Perishah writes that this reason is not sufficient, because if she had waited already 10 years after her husband died, this should mean she has remitted on 10 Sela'im and would only receive 15 Sela'im, which does not conform with the Halachah, which states that she does not lose anything before a full 25 years are up.

Dovid Bloom