I have no idea. It probably varies with the weather. I rode home the other day in "regular clothes" and managed to avoid a sweat at about 14 mph, but it was around 30°F that evening. Even then the sweat kicked in a bit as soon as I stopped riding.

When I first started biking to work I rode in regular clothes all the time, but at anything above about 60°F I ended up changing my shirt when I got to work and hanging it up to dry. To me that kind of defeats the purpose of wearing regular clothes.

Of course, the biggest reason I prefer lycra is the comfort it allows in the groinal region. My non-bike pants are blue jeans, which are just about the right combination of slightly loose and not quite flexible that they'd make a good apparatus for kneading bread dough. That problem is also mitigated if I ride slower.