I've been dutifully pollinating every bloom on the hybrid hibiscus seedling ['Blue River II' x grandiflorus]. First I did a bunch of self-pollinations, and now I'm doing a bunch using pollen from 'Moy Grande'. Since it's so nocturnal, I figure the best chances of success should be right after it opens, which usually is around 10:00 or 10:30 at night - like in this flash picture.
By 9:00 the next morning, the flowers are usually starting to close back up (picture below) and by mid-afternoon they're usually pretty much closed. The open and close timing does vary a lot though depending on the weather. Cool, wet weather delays the opening and also delays the closing.