For some reason, Stack Exchange has it set up so that, instead of waiting for a fixed amount of time (say, 24hrs) before allowing you to review again, it resets your review count at UTC 0:00.
If you do 20 reviews at 6PM EST (for example), then you'll only need to wait 2 hours before you can review more in that queue. If you do them at 5AM EST, then you need to wait 13 hours.