30 minutes is a good idea for me, and disable it in budokai. As other people said is a shit when you get DC with 1 hour cd left, about using CD gear only before transform to reduce the CD i have remember you that not everyone has the money to buy it, until you get lvl 70 and farm (or donate) for weeks you wont be able to get all your set...
And definitely ultimate / gran cheff majins should be and exception, they can´t do almost anything in pve without kid boo, so if you dont want to reduce their CD al least let them to use buffs while transformed.