I help my sister sometimes when she uses her long arm to quilt. Most of the time we can get a quilt quilted in about three hours. It always takes longer than we think, as the tension will get wrong, thread will break, etc. If she has her pattern picked out and the machine threaded, it takes about three hours, even with a few problems. If the top, backing, and batting are already on the machine, it takes less time.
Like someone said, it is a very individual thing, and it depends on the size of the quilt. My sister feels like her machine is only happy if she is sewing a bit slower than she wants to, so that may have something to do with it.
Dina