I seem to recall we went quite early when we've eaten at Inventions (sometimes nice as you get the characters mainly to yourself to start with) but then to be honest, the parks closed quite early on the first trip so it was something nice to do later.
The characters are there until quite late - my DH and I used to sit in the bar for a drink at the end of the night (or the atrium type bit of the bar which is open to the restaurant) and watch the mayhem the characters were causing with the diners and that must have been around the 10 pm ish time. Was fun to watch
Was nice as having gone with Mum, we had connecting rooms so we used to take turns going to the bar or sitting in the non baby room but with the door ajar so we could hear them but still have a TV/drink/bit of a night as the girls were so tired and too little to stay up until all hours. I don't know what we would have done if we hadn't had that 'extra' room - sat in the bathroom talking in hushed tones I guess