The authors surveyed Canadian and American sex therapists views on how long they felt it was normal for intercourse to last. They found that the average sex therapist believes that intercourse that lasts 3 to 13 minutes is normal.
Corty EW, Guardiani JM.
Canadian and American sex therapists’ perceptions of normal and abnormal ejaculatory latencies: how long should intercourse last?
J Sex Med. 2008 May;5(5):1251-6.