It depends. Some sites are based on raked hands only. Others are based on player points, which are usually easy to accumulate at high stakes but difficult to accumulate at low stakes.
is a good example - To get their bonus, you used to have to play just 100 raked hands - piece of cake! Now, they offer a much larger bonus, but it takes something like nine times the bonus amount ($100 bonus = 900 hands) for it to clear. Like though, they release the bonus to you in $10 increments, so you don't have to wait forever to get your hands on it.
Some places will actually give you the bonus money up front - you just can't cash it out until you earn it. I'm pretty sure is still this way, and I know is. They'll give you $10 for free PLUS a 40% deposit bonus up to $100. That's not a bad deal. I didn't happen to like the site very much, but I'm certainly not above bonus whoring. I love free money
They are having a $100,000 free roll soon too....
-Mark