If you ride some quality 1.5" road slicks on the road, you should get a few thousand miles out of them. It depends on your weight/load/style and how good they are. There is no point riding knobblies on the road, they suck, and semi-clicks are for mixed riding on road and trails.
You know they are really worn out when threads start to show through the rubber. Try and replace before wear gets to that stage. You dont really need the grip pattern, but it is a useful wear indicator.