To reinforce whats been said, I'll paraphrase the advice I(and a few others) got a few years ago from Greg Raymer on 2+2 about rebuys. If you aren't prepared or willing to take every rebuy and add-on to which you're entitled, don't play.
The 7x figure is a solid rule of thumb, though I consider 5x to be my average. The key here is to understand the texture of your table. At a wild table full of rebuy maniacs and little fold equity, you can get away with playing fairly tight as long as you're willing to also take advantage of all +EV pot odds situations (like preflop it's obvious you'll get more than 3:1 for all your chips so you can call with something like 97s).
On the other hand, if you're at a tight table, where it looks like fold equity is relatively normal, *you* have to become the rebuy maniac. Or at the break, youll be lowest in chips, and be in serious trouble.
Anyhow, the turbo rebuys are enjoyable, because they're wild early and super-tight late, so if you can get to where there's about 2.5x players/seats left you can cruise to the seat by stealing intelligently.