Depends on the dungeon(some have more artificial breaks then others), and it depends on the team. A dungeon that doesnt let you even try to pull more then 2 groups at a time with a team that is weak on aoe and not necessarily playing their classes well? Thats probly a 35-40 min run(closer to 60 if theyre also wiping to boss mechanics). A dungeon that you can not only do big pulls on but easily survive them, with a strong aoe group and a tank and healer on top of their game? They might push 10 mins, and will certainly be done in less then 15. The 18-22 you mention is a normal sounding range for average groups(as in less then exceptional, but better then awful) in normal dungeons.