Depends on the plane and my mood. How badly do I want to fly!

Most of my parkflyers come down at 10 mph
Aerobird goes to about 15 mph if I really want to fly.

Sailplanes go to about 15 if I really want to fly, but finding and holding a thermal at that speed is hard.

Slope planes are just getting warmed up at 10 mph. 20 MPH is great! At 30 MPH the lift is great but my skills are being challenged with my current ZAGI 3C. However, at 30 MPH the lift is so strong I can almost jump on and ride!!!!!