Board Thread:General Discussion/@comment-24382631-20140105033403/@comment-8986886-20140105140920

Xilinoc wrote: In Ichigo's case (at least with his Quincy powers-powered Bankai), he can maintain it for at least 3 months straight because all of its power is compressed into a small form, so take that as you will.

You must be referring to Ichigo's Bankai training in the Dangai, to learn Saigo no Getsuga Tenshō. But even if Isshin stated that he could control the Kōryū for 12 weeks, using Kaikyō Kotei, it's no guarantee that Ichigo was able to maintain Tensa Zangetsu for that long. For all we know, he could have rested from time to time during that timeframe; if not, he would probably have been as exhausted as Isshin were, when Ichigo confronted Aizen. ^^