It all depends, really. If in a group, I can do 25+ mph on a flat, windless road for 2 hours......or at least I assume I could as I've never found a road like that yet.

If I'm doing it solo, I'd estimate going at it about 20-22mph depending on how I felt.

Most rides I do involve quite a bit of wind and a whole lot of hills. That and riding mostly solo means that I'm lucky to average 16 on most of my rides.