Technically it's not so much a time aspect (outside of the first 30 minutes of a post being created) but rather how many others have voted before you. The curation reward percentage do go down rather sharply, especially after that first big Steem Power vote gets applied.
While spitballing a number for you...after that first whale (high sp voter) it'll drop about 70-80%. The goal is to get your votes in early, before others, on a post that you think will be popular.
Thanks a lot! :)