how long can the police keep my firearm before they have to return it?

The police came to my house one day and said that someone called and reported that they're house was struck by a stray bullet. They said they had heard us shooting Guns throughout the day which we were. We didn't shoot the direction of that house at all and we were shooting pistols and this house is almost 1.5 miles away so I don't believe the round came from my gun. I gave them my pistol when they asked to have it to match it with the slug from the house. Now they are refusing to test it and just want me to say who all shot the gun so that we can ask get a ticket for it. If it would match my gun I would gladly tell but they won't match it up. How long can they keep it before they have to test it and return it if it doesn't match?