I have some time to repaint the scarab this month but have people coming up and will be wanting to run it quite a lot.
How long do I have to leave it out of the water after it's painted?

You don't have to. A day or 2 would be nice. The hotter the better, and let that sucker bake in the sun for a couple weeks after paint. But you can use it almost right after paint but to get full durability and curing complete she need time and sunshine.