I guess some real high stakes players could theoretically live on
rakeback, they probably receive several thousand a month on rakeback alone. That being said, I'm sure that those players take a nice little chunk out of their winnings each month to boot. If they ended up on a losing streak, the rakeback needs to be put on the poker account before they end up playing at a whole lot lower limits.
As an example, I've got a great
rakeback deal (won't go into details about it), and I usually get around 2-3 times the max buy-in on the level I play and I don't play insane amounts each day. Basically it's possible to get thousands of dollars in rakeback and leave the winnings in one's account, but still... don't see that happening. Highly theoretical.