We did like 3 dolmens in 10 minutes for 5h during last festival, so I would say that 15 minute dolmen respawn time is highly unlikely. Maybe it is like that when you do them alone..
There is always one dolmen up in the zone. There is no respawn time. Just if one dolmen is destroyed, another will activate. So there is no reason to wait on one. Go and fight on another
That being said, I did get into a Clockwise rotation in Alikir the other night, so perhaps the rotation can be influenced by the players.