Now, first of all it should be dependant and up to the sides. It should not have any limit. It is a correspondence chess, it is between me and the adversary: the post (chess.com) should not have a say in our vacation time, we are the players and we should decide how much we can go. chess.com serves here as a post through which I send and recieve my moves. It can not have a say in amount of time I can delay my move, my adversary can have it. Now you may say what if I go to vacation till eternity? As I said, it should be dependant: the sides must set a maximum days of vacation in advance to the game. I want to play a casual daily game, when I issue a challenge I shall set the vacation time like "up to 20 days at most." That is all. When my opponent surpasses that limit, he loses. Likewise, when I am scanning the challenges I will accept in a similar way and when I do not comply to it, I will lose the game.
In team matches (like chess.com world league), the match organizers will set the vacation time like all participants of this match can 10 (or any amount) days of vacation time.
And Here is my proposal about daily chess time control (novelty): https://www.chess.com/clubs/forum/view/proposal-new-type-of-daily-chess-with-only-a-specific-time-for-the-whole-game-shall-be-used
It is entirely different.
I think it would be nice if you made it customizable, in tournaments and club challenges. Now we only have Vac or No Vac, instead it may be practical for a TD or team admin to be able to chose a vacation limit for the event when he creates it...