I can tell you at least how it works, and you can decide on your own strategy with that info.
Curation rewards on an upvote at 0 minutes are 0%, and at 15 minutes are 25%. That's the max, and they go up proportionately in between (12.5% at 7.5 minutes).

There are some good posts by @miniaturetiger out there about the nitty gritty of curation. My simplistic advice is to never upvote before 7 minutes, because you'll just lose too much to the algorithm. After that, and up to 15 minutes, it's a guess about when the big votes will come in. You're trying to precede the big votes by just a little (though after 15, it doesn't matter by how much you precede them).

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