Usually I wait at least a day or two before putting up a demo. It doesn't take long to figure out if it sounds good- how much time do you need to spend with a cheeseburger before you know it's tasty?

It depends on the complexity of the pedal- I never put up my planned review of the Octavius Squeezer when I got it because it took me weeks with it to really get into it, and at that point I knew it wasn't for me.
Flipping is a whole different thing. For almost everyone, it's a case of either needing the money, or no matter how awesome what you have is, the grass can always look greener on the other side.