I ride with one of the largest and best cycling clubs in the country. Our regular Saturday rides break down as follows:
No Drop group - Typically 12-14 MPH over either 25 or 40 mile route. If you cant maintain 12 mph, you will be helped back to the park, but probably shouldn't come out to the group ride until you can. (I started here, after riding solo for about 6 months, loosing weight and getting stronger.)
14-16 MPH - 40 miles (rode with this group for about 6 months, till early this year.)
16-18 B - true average, 40-60 miles (I usually ride here... clyde, have weight to loose, but pretty strong at this point.)
16-18 A - Liars club. Anywhere from 18-21, usually 40-60 miles (I sometimes ride here when feeling feisty. Its tough and a bit competitive.)
18-20 - This is where the junior race team rides, fast weekenders (I'm going to try this soon just to see if I can hack it.)
20-22 - Serious amateurs looking to ride with the big boys, race team guys who are in need of a steady state ride
22+ - Mostly race team, quite competitive even though its not a race
Theres your benchmarks.