I usually keep cats about 2 weeks as I haven't got the room or the money to keep any longer. I did take one in a few weeks ago from the vets, he'd been hit by a car and had to have an op on his jaw. He was in there approx a week and they had no enquiries. I took him on, kept him a week, rehomed him and the owner came forward. Luckily for them, the new owner understood and did let him return. I think part of the problem with some people is their cat disappears, they think they've been run over or got lost and they won't come back so they just accept it. The only reason the lady got this particular one back is she'd got talking to a neighbour a couple of weeks after he'd disappeared and she said that a black/white cat had been run over round the corner and someone had taken him to the vets. I do think 2 weeks is long enough to keep a cat, I usually inform all the local rescues, vets etc when I have one handed in in the hope that an owner will come forward.