Kind of short for a game that actually costs money, but since I never donated before I'll just look at it like I was paying for all the fun I've had with his previous games at the same time.
That's the way I looked at it, too. I think paying 19 bucks as thanks for the many hours that Saira, Knytt, Knytt Stories, Within A Deep Forest, A Strange Dream, etc. is perfectly reasonable. Particularly if there'll be further episodes yet to come. It's like a debt paid and an investment all in one!
As for me, I sat down around 2 PM to play the demo, bought the game around 3 PM and finished about half an hour ago (shortly before 6 PM). All in all, it was a great afternoon!
Saw all 6 endings, and managed to spot a couple of the secrets they're talking about over in the Secrets thread. I might try fiddling with the F12 debug later this evening, seeing what fun stuff I can find!