http://www.fulltiltpoker.com/deposit-bonus-details.php
For every $1 in rake you participate in (not sure about FTP defines 'participate' but some sites define it as merely being dealt cards, while others define it as calling a pre-flop bet), you earn 1 point. Each point you earn releases 6 cents. Therefore, to be able to release your $50 bonus, you must participate in $50/$.06=$833.33 of rake hands. The maximum rake per hand is $3; therefore the very minimum amount of hands you must play (at full rake of $3 per hand) is 278 hands (833.33/3). This amount of hands will always be a little more because not all hands are raked (no flop no drop). There is nothing to prevent someone from playing the absolute minimum amount of hands required and taking the bonus money (in addition to whatever bankroll you have left) and withdrawing that amount, because this minimum hands safeguard is already in place.
But as already mentioned in this thread, for those playing a decent amount of hands per month, rakeback is almost always more profitable than bonus hunting. Many sites offer 27% rakeback for FTP which means for every $3 of rake you pay, you get 81 cents of that back, versus 18 cents per hand max for the signup bonus. It should be noted though, that for the purposes of rakeback, it is almost always defined as the amount of rake you pay for the hands you've won. That means if you lost the hand, you paid no rake and get no rakeback.
The greatest thing about rakeback is that if you are a break even player, everything you earn through rakeback is pure profit. It is a very powerful means to not only recoup your loses, but also to accentuate your earnings.
P.S. If you are interested in earning rakeback at Full Tilt Poker or any other poker site, shoot me a PM and I'll see what I can do.