Here's from the Steemit FAQ
How is the reward pool split between authors and curators?
Up to 25% of a post's payout is awarded to curators (the people who upvoted the post) as a reward for discovering the content. The other 75% is awarded to the author. If curators vote for a post within the first 30 minutes of it being created, a portion of their curation reward is added to the author payout. This portion is linear to the age of the post between 0 and 30 minutes. Therefore upvoting at 15 minutes old will donate half of your potential curation reward to the author.
Interesting to me, this says the opposite of what I said, that it's better to do it right away!