We have a 7-day money back guarantee on both new and used and a 30-day exchange policy on used. It does not get used very often. Averages about once a month. It's truly just peace of mind. I don't want a customer in a car they decided they don't like or don't want. I believe allowing this creates more repeat business than it loses business. It's also a good closing tool for "I need to think about it."
Fun Fact: We have a written policy, but we did not used to provide a copy of it in the deals. We decided that we maybe should, so we started including it and having the customer sign it to acknowledge it. They, of course, never actually read the policy. Yet there was an immediate and obvious increase in our returns/exchanges.. At the slightest hint of trouble, people would talk about bringing the car back. This was definitive proof that there is some psychology to a deal being finalized. Reminding the customer they had a way out right at the end of the deal basically took the commitment out of it. Ultimately, we decided that it wasn't in the customer's best interest, because they really didn't WANT to bring the car back, they just thought it might be the only recourse. I would recommend having a clear policy, but I would not make it part of your deal jacket.