i once traded an Epi Dot the same day i got it, but it was for a more valuable piece of gear, that i then traded for a Martin. Best gear decision i have ever made hands down. It was a great guitar just not worth turning down that trade, didn't even give it a chance to bond with. Dots are a dime a dozen though so screw it.
Depends on the piece. A pedal I can make the decision almost instantly. Guitars It depends on the neck and playability. If it feels good I'll give it a few weeks , months to make it sound better if it doesn't sound great right away. An amp......I could figure it out within 48 hours tops. Sometimes its best to just put a piece away for a while and come back to it. I've made harsh decisions in haste that I've really regretted.