ART!
Deluxe Unhuman
The players level up when it feels right in the story. We slow it down around 4th level because no one in our group is a fan of flying up too quickly.
That's basically what i do. I tend to throw in a deadly encounter or the like before giving them that next level, though, or if an encounter winds up unexpectedly being incredibly difficult or deadly, I might have them level up before the next session.