[Hoss1193]

Since you didn't specify, my comments apply to live game Limit Hold 'Em. Here's what I do, and it has worked well for me.

I buy in initially for 25 BB ($200 at 4/8 limit). Well more than half the time, this is plenty; I won't have go dig any more.

Adding another 12.5BB ($100) is pretty automatic for me if I get down around $75 or so. Reason; I try to never play with a real short stack. Finding yourself all-in on the turn in a multi-way pot while holding the stone-cold nuts is just poor money management, in my opinion.

I'll buy the 4th rack (again, when my stack gets down to around 75) only if I'm quite sure that I'm playing well and still have an edge. Usually this is the case, but not always.

4 racks is about it. I don't think I've ever bought a 5th rack...not sure I was ever faced with a decision to do so.

So I guess all this rambling boils down to: 50BB. And if you don't want to rebuy, then buy the whole 50BB up front; you might get some funny looks, but who cares? If you get down to 10-15BB, however, stop and take a hard look at your own play, table conditions, and make an honest decision on whether to keep playing or to save your remaining bets for another day.