The ideal time to upvote is before everyone else.

Curation is easily the most complicated component of the rewards system. Typically, 25% of your upvote goes into a curation pool that is distributed based on a time weighted average. Earlier is much better. If you upvote before thirty minutes, you give some of your upvote to the author instead of the curation pool. That decreases your stake in the pool, but can get you favor in the time weighting.

In HF 20, that changes from 30 minutes to 15 minutes. So an upvote at 15 will put 25% of its value into the curation pool for the post instead of only 12.5% now.