| It really depends on a lot of other factors.Rooms and Corridors generally are usually low polygon - a wall could just be 2 tris if the whole wall shares the same texture.|
The level builder in Quill3d is very good at keeping level geometry low poly.
If you have cut-up the level using CSG for doors and windows then this will also increase the polycount.
If the rooms contain a lot of static meshes and/or other things like particle generators then this can increase the amount of tris that need to be drawn and will need to be taken into consideration as well.
I would say that a level of 6 rooms joined by corridors is a relatively small level but it does depend on just what it contains. If it is pretty much empty with simple geometry then it would be a very small level ( in terms of polygon count) indeed.