That 20 minutes I think is there to vote at that same exact time after they post. Here's a good article that some discussion on when to vote.

https://steemit.com/steem-help/@liberosist/mind-your-votes-an-investigation-and-guide-to-maximizing-your-curation-rewards

The official FAQ also has a good discussion: https://steemit.com/steemit/@shenanigator/thanks-to-your-help-the-official-steemit-faq-is-now-complete