Users are allowed to up/downvote up to 40 posts per day (with the supply refilling at 0000 UTC). However, there seems to be a bug regarding this; for each of the past several days, I've been getting the "Daily vote limit reached; vote again in [xxx] hours." message after only 37 or 38 votes, which is kinda frustrating when it means I'm left with a couple good questions and/or answers that I have to wait another day before I can upvote. Why is the votes-used-today limit triggering prematurely?
This is because the daily vote limit is actually 30, not 40, but you get a maximum of 10 bonus votes, one for each question you vote before you see the "N votes left" warning. This is to encourage more voting on questions.
For more details, see the question Cannot vote on question although 40 votes limit not reached on Meta Stack Exchange.