In general, the legal limit for flying drones in most places in the U.S. is 400 ft. above the ground. This is quite sufficient for most purposes. In some cases, that limit may be lower due to FAA limitations such as near airports. In such cases, I have developed techniques to get images of large areas even while staying below 100 ft.
For some photo missions, this limit can be much higher, specifically when being within 400 feet of a structure, I can operate up to 400 ft above that structure. This allows photography and inspection of tall buildings and things like towers.