Gemara

(Mishnah): A Nazir is liable for eating a k'Zayis (the volume of an olive) of grapes. The initial version of the Mishnah said that one is liable only if he drinks a Revi'is (the volume of one and a half eggs) of wine;

R. Akiva says, even if he soaked his bread in wine and ate a k'Zayis, he is liable.

Yoma 80a: One who drinks (on Yom Kipur) Malei Lugmav (is liable).

(Shmuel): 'Malei Lugmav' does not literally mean his full mouth. Rather, it is enough that if he would tilt his head to one side his cheek would appear full.

Kerisus 12b (Mishnah): If one entered the Mikdash after drinking a Revi'is of wine or Tamei liquids, if he drank within Kdei Achilas Pras (the time to eat a half-loaf, the volume of three or four eggs), he is liable. (For wine, he is liable mid'Oraisa.)

R. Eliezer exempts if he paused while drinking the wine, or added any water to it.

13a (Beraisa): All (Tamei) drinks join to the Shi'ur of a Revi'is to Metamei (mid'Rabanan) one who drinks them bi'Chdei Achilas Pras.

Rishonim

Rambam (Hilchos Shevisas Asor 2:4): If on Yom Kipur one drank intermittently bi'Chdei Shtiyas (within the time needed to drink a) Revi'is, the amounts he drank can join to the Shi'ur of Malei Lugmav.

Source #1 (Ran Yoma 3a DH ul'Inyan (2)): The Rambam learns from the Tosefta (Pesachim 1:10) which says that regarding Chametz, liquids join to the Shi'ur of bi'Chdei Shtiyas Revi'is. However, Tamei liquids join to Metamei one who drank them bi'Chdei Achilas Pras!

Hagahos ha'Gra (on Tosefta Pesachim 1:10): The correct text of the Tosefta says that they join bi'Chdei Achilas Pras.

Source #2 (Magid Mishnah): The Rambam learns from the Tosefta (Yoma 4:4). The same Shi'ur applies to other Isurim. Some say that liquids join bi'Chdei Achilas Pras, the Shi'ur for Tamei liquids. It is better to say that Chachamim were more stringent about Tum'ah mid'Rabanan, so we need not reject the Tosefta.

Question: The Gemara (80b) says that we are more lenient and allow less time to join foods for Tum'ah mid'Rabanan!

Answer (Lechem Mishneh): Regarding Tamei food, Chachamim were lenient and allowed only as much time to join foods as is given for (liability for) a smaller quantity of Isurim. They were stringent to equate the Shi'ur for Tamei liquids to that of food, lest people treat both Tum'os lightly.

R. Mano'ach: Drinking the Shi'ur brings relief (to one was fasting) only if it was bi'Chdei Shtiyas Revi'is. For a Zar to be liable for drinking Terumah he must drink a k'Zayis, like the Shi'ur for eating, bi'Chdei Achilas Pras (Tosefta Terumah 7:3). One becomes Tamei for eating or drinking the same amount (according to the Rambam, half a Pras equals a Revi'is) of Tamei food or drink, therefore the Shi'ur of time is the same (Kdei Achilas Pras).

Minchas Bikurim (on Tosefta 4:4 DH Kedei): The Halachah is that we join what one drinks bi'Chdei Achilas Pras. The Gra corrects the text of the Tosefta to say Kdei Achilas Pras.

Rambam (Hilchos Terumos 10:3): If a Zar drank Terumah and paused in the middle, he is liable if he drank a k'Zayis within Kdei Shtiyas Revi'is.

Ra'avad: Indeed, this is like the Tosefta in Terumah. However, the Gemara in Kerisus and the Tosefta in Pesachim allow Kdei Achilas Pras.

Question (Radvaz): It is more reasonable to follow the Tosefta in Pesachim. Since the volume to be liable for drinking is a k'Zayis, the same as for eating, presumably the time in which it must be consumed is also the same!

Answer (Radvaz): It is more reasonable to learn from what was taught regarding Terumah itself. The Rambam learns that in general the Shi'ur for drinking liquids is the time to drink a Revi'is.

Poskim

Shulchan Aruch (OC 612:10): If on Yom Kipur one paused while drinking, if the total time was at most Kdei Shtiyas Revi'is, we join what he drank for a Shi'ur. Some say that the time is the same as for eating, Kdei Achilas Pras.

Gra (DH v'Yesh Omrim): The Mishnah and Beraisa (Kerisus 12b,13a) clearly refute the first opinion (the Rambam)! Perhaps the Rambam learns from R. Elazar, who exempts if one paused while drinking the Revi'is or added any amount of water. Both are exempt for the same reason, because he did not drink a Revi'is of wine bi'Chdei Shtiyas Revi'is. This is wrong. R. Elazar says so only regarding wine, for it says "v'Shechar" (an intoxicating drink).

Shulchan Aruch (190:3): One should drink a full Revi'is from the cup of Birkas ha'Mazon to enable saying a Berachah Acharonah.

Shulchan Aruch (210:1): If one drank less than a Revi'is of wine or any other liquid, he does not say a Berachah Acharonah.

Mishnah Berurah (1): Some say that if he paused in the middle, he does not bless afterwards. The Gra holds like the opinion that it joins, as long as he did not delay more than Kdei Achilas Pras. Normally, tea and coffee are sipped, and one does not drink a Revi'is straight. Some say that according to the first opinion, one does not bless after them. Meticulous people allow a Revi'is to cool off so they can drink it straight and bless afterwards.

Kaf ha'Chayim (190:18): Most Poskim require drinking bi'Chdei Shtiyas Revi'is, like regarding Yom Kipur. Further, if one delayed longer it is a Safek, and Safek Berachos Lehakel. It is best to drink another Revi'is without pausing, to enable saying a Berachah Acharonah. If this is not possible, he should think the Berachah in his heart.

Shulchan Aruch (271:13): One must drink from the cup of Kidush Malei Lugmav, i.e. so much that if he would put it in one side of his mouth it looks like his mouth is full. In an average person this is less than a Revi'is.

Question: The Gemara explains that the Shi'ur for Yom Kipur is only k'Malei Lugmav (to fill one cheek). Why didn't it explain this regarding Kidush?

Answer (Drishah 3): Regarding Kidush it was obvious that a full Malei Lugmav is not required, for this is more than a Revi'is in an average person, and the cup requires only a Revi'is.

Mishnah Berurah (68): It suffices to drink a Revi'is even if this is less than Malei Lugmav (e.g. for a giant). One should drink Malei Lugmav without pausing much, i.e. bi'Chdei Shtiyas Revi'is. Certainly one must not delay Kdei Achilas Pras. If he did, b'Di'eved he was not Yotzei. People who make Kidush on whiskey are prone to transgress this.