not long at all to spin up to 20mph sustained on level ground. The big key is your position in regards to your crouch. But if you have wind at your back, just lean up. I tend to ride on my largest chain ring until I can't so I reach 20 and sustain it for maybe half a mile or so on a regular basis. (that was when I had my flatbars on the bike) Since I have changed to the drops, it's a whole other story. To me a half mile is not really sustaining a speed though. Around here, there is not enough flat land to sustain anything anyways.