dpends on time of year
I start using mine in Aug. I leave it in an area that I have reason to believe is frequented by a big buck (another thread). If I don't get him in 2 weeks, I move it until I find something I want to hunt. If I get really nice deer on it, I laver it there and visit it very infrequently. Leaving it there 2-3 weeks at the time strings as little scent around as possible. If I get a pic of a buck and don't see him on the camera again, usually, I will put it back in that spot a couple of weeks before the rut and leave it there. Just before the rut gets into full swing, I'll check it to see what time of day he's there, which way he's entering the picture fro, etc. Is he muddy because he just came from the swamp? Does he have briars caught in his horns because he just came out of the cutover? Does he have cockelburrs stuck to him from the burr patch down the creek? All of these are clues and if he's not at my camera during legal shooting time, they will help tell me where he might have been during those times.
Keep the camera moving every 2 weeks or so. Let it sit longer if you start scouting earlier than I do. Don't check it more than once per week. Lessening your intrusion is more important than getting the pics every week. Besides, if you are taking pics of a big buck, you need several days worth of pics to really pattern him. Just because he's there at 4:45 today, doesn't mean he'll be there at that time tomorrow. You want to get an average time he's going to be there and find out how he adjusts his movements and feeding patterns to the temperature, weather, etc. There's no harm in leaving the camera in a spot for 2-3weeks. However, if you're not seeing any sign, or getting any pics, move on.
Again, don't worry too much about loosing a buck you got on camera. Do your scouting. If you think he's coming back, reinstall the camera in that spot during the pre-rut. Deer don't live by any set rules. Therefore there are no set rules for how long you should leave the camera there. If you see sign, but don't get a pic, move it to the sign, but be patient, you still have pletny of time to get up with ol' mossytines before deer season. Most inportantly, don't interrupt his pattern before the season comes in. Stay out of there as much as possible.