User blog comment:Tensa gestsau/Hollowified zangetsu (bleach decide 11)/@comment-2129921-20100625185855

Ichigo can only maintain his mask for a certain amount of time, the amount of time required to enter your zan and defeat the spirit inside hasn't been disclosed but if Isshin felt Ichigo needed 3 months to learn one technique I doubt he would be capable of keeping his mask on that long.