I'm an electrical guy, not a mechanical guy, and I've never built a robot so you can take this with a brick of salt! 
Practically speaking, I don't think you can calculate it... I think it's more a matter of experience (and experimentation). Nobody knows exactly how fast a race car will go 'till you get it out on the track!
The problem is... speed is limited by friction. And, I don't think anyone can predict the friction with a robot. If there is no friction, there is no "speed limit" and a tiny motor could easily get you up to 2MPH (after accelerating for some period of time). If there is zero friction, it only takes force to accelerate an object, but zero force/power to keep it moving.
Also, (most) electric motors have a torque-speed curve that has maximum torque at zero speed (or near zero), and minimum torque at maximum speed. So, although friction is fairly constant (until air-drag becomes a factor), the torque drops-off with speed.
If you have a gear-motor, there is friction in the gears. Otherwise, most of the friction is between the wheels and the floor. And, I think the starting-friction is higher than the rolling-friction.