I believe the "hot" page is till 8 hours of the post's age. Depending on the post's value and the age (0-8 hours) it determines how high you are on the list.
Waiting 15m is not exactly a half/half split, it means you get 50% of the potential curation reward pool (Which I believe is 25%). This means that after 30m you get the full curation reward, but if someone else votes before you with significant SP (compared to yours) you barely get any curation rewards at all.