I'm guessing that you mean something like "how fast should I be going to get better/faster in an efficient way", and I would second alpinerabbit's idea of going by heart rate or perceived exertion. Think of it as a workout, keep up a rhythm, don't coast, keep pedalling, mix it up with intensity spurts up hills and easier parts on the flats. If you're tired afterwards, you worked hard. If not - kill those hills a bit harder next time.
I have quite different average speeds on a ride depending not only on which bike I'm using, but what the terrain on a specific ride is like, and which way the wind is blowing, and if I'm having a good day or not... so really, it's impossible to tell you what speed you "should" be aiming for in general. Measure yourself on the same route every now and then, and set yourself a time goal, if that's your thing.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett