After reviewing 20 close vote duplicates I got the "Thank you, come back in X hours" message. I'm curious as to why the number of hours varies. Yesterday I got one that said 6 hours and today it says 9. Why is this variable and how is it calculated?
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. I guess it's easier to reset everyone at once rather than have separate timers for every user.
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.
Your counter resets at UTC 0:00. So, if you completed review at 22:00 UTC, you need to wait 2 hours. If you did at 1:00 UTC, you need to wait 23 hours.