If it's within the first 15-30 mins, I'll usually rebuy because the blinds are still reasonable as are the opponents chip stacks. I'll rarely rebuy outside of that period, unless there is overlay galore or its a promo tourney with other benefits or gtd's. After that 30 min mark, I find I'm just wasting cash most of the time and putting myself in a position where I have to make marginal calls and hope for the best.