How long after you sign papers for buying a used car do you have to change your mind?

Answer 1

Presuming no special dealer/seller return guarantee and that the car doesn't qualify as lemon under the lemon laws of your State (which all require a lot of proof and an opportunity for the dealer to correct), that as you say, you just don't want that car now:

There is no time or limit on when you can return it. That's because, there simply is no returning or reneging on the sale. It's bought, it's yours. Done deal. You made a contract and deal, presumably because that's what you wanted to do then. Nor can the Seller take it back or decide to charge you more for any other reason, like he has someone willing to pay more sometime later, or it didn't blow up as he thought it would and it must be better than he expected.

Of course, you may now sell it...and you may get much less (or more, if you made a good deal or fixed things), than you paid for it. That's the cost of having use of it and having others act on your wishes and then changing your mind.

However, if you have only signed the contract (read it!) but haven't paid, while it may not be legal required, the reality is that you may be able to get out, (probably lose a deposit), by refusing to come across with the money.