Today I reached my "daily" limit of votes. I was told to wait one hour before I could vote again. Last time I had to wait four hours. Why?
Votes roll over at a set time; specifically, 0:00 UTC. The timer for resetting your daily vote limit is counting down to that; today, you ran out of votes at 23:00 UTC. Previously, it happened at 20:00. That's all.
To see the current time in UTC and adjust for your time zone, consult one of dozens of highly ranked google search results.