Board Thread:General Discussion/@comment-24382631-20140105033403/@comment-5398287-20140106080518

Well, here's the problem in providing an answer to this question: almost none of the Bankai users in the series have ever run into a time limit for them to keep their Bankai active or commented on such a time limit being there, and in Hitsugaya's case it was due to his Bankai not being complete, something which he supposedly remedied over the timeskip.

So as of right now, there's not really a way of knowing what affects the length of a Bankai's release.