I review until it tells me to stop. "Come back in 6 hours" Then I get busy with other things and don't always make it back.
Edit: It occurs to me that some may stop reviewing when there is no longer a perceived benefit to them! I think that we all understand the benefit to the site as a whole and that's the reason I do it, but there's no rep to be had and no more badges once you've done 1000. Because of these facts I can understand why many would feel there time is better spent answering questions than "pushing the mop" ;-) . Perhaps an incentive of some kind would help. That may be a new related question. Should there be more incentive for working the close vote queue?