definitely overnight, sometimes longer.
What sort of milk did you use?
I hang mine (goats milk/cheese) to drain from a kitchen cupboard door handle, I'd have thought the airing cupboard would be far too hot?
It really takes ages to make, pasturising, cooling, standing, draining, but is worth it in the end! (the cooling part is the worst for me, it takes ages and I hate to leave it in case I forget and it gets too cold - then would the starter still work?!)
Well done on the butter making, its not something I've tried (yet)