7.5 miles direct (main roads, flat)
10 miles on National Cycle Network equivalent (no thanks!)
10 miles if I want to add a couple of hills
17 miles for the scenic route (mainly country lanes)
I usually go for the direct, road route, especially in the cold/wet/dark. The hilly route makes a nice change. I save the scenic route for 'best'.
Oh, and the National Cycle Network route... that's for if I'm feeling masochistic and want to spend 50% longer on my commute so I can negotiate dogs in the park, schoolkids walking in the cycle lane, cars reversing out of drives, give way at every side-road, wait for crossings, get off and walk over the canal bridge, etc, etc...