There are a few things to take into account when it comes to curation:

1. The first 0-30 minutes your curation reward is reduced from 0-100%, so generally u want to not upvote too early. This will change in HF20.
2. The earlier you upvote the more of the curation you receive. So it is good to upvote before the whales vote for a post.
3. overall about 25% of the reward of the post goes to the curators. How much everyone gets depends on the SP, or upvote value (rshare) and the two other points above.

In summary in order to make your votes profitable you want to upvote content that will be popular before everyone else, close to the 30 minute mark. (15 minutes after next hf)

This is the very rough view overview. Anything more detailed you have to look at the code, as the actual algo is quite complex.