If you have good enough credit, the dealership should be able to give you an answer within minutes, either through an automatic approval or with a quick response from the credit union.
Since you mention that you have a cosigner, I can only assume that maybe your credit is not perfect. Two things about a cosigner. If your credit is thin, they will look at the cosigner to bolster your credit. If your credit is really bad, it may not matter how good the cosigner's credit is. If your credit is really bad, you need to put your cosigner first on the deal and make you the second signer. He or she will still be on the hook, but at least you can get the car.
If the deal is shaky enough that the CU has to look at the deal, it should usually take no more than an hour to get an answer. If the CU is really busy, maybe as long as 2 hours, but I have never seen a CU take longer than 2 hours to give at least some sort of response, unless they are closed.
If it is taking longer than 2 hours to get a response, the most likely thing that is happening is that the CU turned you down. In that case, the dealer is probably sending your deal out to any number of sub-prime lending companies to try to get you some kind of deal. If that is the case, you need to watch out that the dealer does not turn around and jack up your interest rate.
If you do get approved through someone other than your CU, I suggest strongly that you tell the dealer that you need to have the interest rate at least 2% lower than whatever they quote you. If they say they can't, walk out, and see if they chase you. If they chase you, you will get the lower interest rate. If they don't, you may have a problem getting any deal.