Why does it say "ka'Aseres ha'Yamim", which implies that they are known?
Rashi (from Shochar Tov 26): Before killing him, Hashem waited the seven days of mourning over Shmuel before making Naval sick, lest the mourning over a Rasha mix with that of Shmuel 1 . He died after three days of illness. It says "va'Yigof Hashem Es Naval" - death after three days [of illness] is Magefah.
Rashi citing Rosh Hashanah 18a #1: These are the 10 days of Teshuvah - Hashem waited to see if he will repent. 2
Rashi citing Rosh Hashanah 18a #2: He lived another 10 days as reward for feeding the 10 Sheluchim that David sent to him. 3
Radak: It was after 10 days. This is like "kemi'Shelosh Chadashim" (Bereishis 38:24). He was sick for 10 days, and died. The afflictions with his death were the punishment for his sin.
He could have become sick after four days, and died three days later, and the mourning over him would not mix with that over Shmuel! Perhaps he lived three days because he had some regret; if not, he would have died the day he fell sick, i.e. death from Divine anger (Semachos 3:9, Yerushalmi Bikurim 2:1. - PF)
Radak: This is difficult. His heart died, and he was like a rock - how can such a person repent?! (What is the source that this persisted all 10 days? Also, he should have realized that it was proper to give the gift, and ceased to be pained over it! - PF)
Radak: This is difficult. These 10 days were for his evil! (This is like Radak holds, that all 10 days he was sick, and like a rock. Refer to 25:38:1:4, 25:38:1:2*. Semachos 3:9, Yerushalmi Bikurim 2:1 - death after seven days of illness is of dearness, and after 10 days is of afflictions. Also refer to Shemos 15:5:1:1 and the note there. - PF)
