If your vote is worth less than 0.01$ the answer is: Whenever you like.

If you are actively trying to maximize curation rewards, the answer is: Prior to big voters upvoting what you want to upvote. Keep in mind that votes cast before the first 15mn period do not provide serious curation rewards to you (edit: the answer below has the table). Curation rewards increase gradually up to the 29th minute and are maximized from the 30th minute onwards. But if more people have voted at that point it may be preferable to have voted earlier (say 15-20-25 minutes) as long as you voted earlier than them.

This system may appear "complex" because it doesn't have a fixed answer, yet the rationale behind it is to reward those who are able to "spot" good content earlier than others. So there is an increased amount of relativity on the aspect of who votes and when they vote.