There is a time limit, but it all depends on what length you pick (I think they go in 30 day intervals). After that your access to videos (and/or quizzes) will expire, aside from the ones they let you watch for free.
I purchased all his videos and I think it was definitely worth it. His gen chem and organic chem are great of course, but I found his math and bio reviews helpful too and his quizzes for all the subjects are good for checking your knowledge. If I recall correctly there's not that much of a price gap between the DAT bundle for videos/quizzes and all the chem videos separately, so I'd go for everything.