Replacing a roof, whether on a commercial or residential building, is a labor-intensive project and, depending on the type of roof, could take anywhere from a few days to 2 or more weeks. The time involved is substantially affected by the weather, as well. Wind, snow, rain, or even just the threat of one of these and will slow the process considerably.
For built-up roofs, removing and replacing the roof will probably proceed at a rate of approximately 1,500 square feet per day. For single-ply roofs, the rate is closer to 2,000 to 4,000 square feet per day.