It varies a little depending on the motor (slightly different motors have been used in different flywheelers over the years, sometimes even across revisions of the same blaster). Stock Barricade motors are just under 38kRPM at no-load on 11.1V and most motors from semi-autos should be somewhere around that - possibly a little lower for motors from something with a 4-cell tray like a Stryfe or Rayven. RS motors apparently achieve around 30kRPM+ on 2S.
Speed under load varies wildly depending on the individual motor and the load applied although stock flywheel motors shouldn't drop below 21kRPM during firing on their respective optimal pack voltage, at least at relatively low RoF (at 21kRPM the surface speed of the flywheel is approximately 120fps or approximately 37m/s - if it did drop below that during firing it wouldn't be running "supercritical" anymore and the muzzle velocity would be limited by the flywheel speed rather than the flywheels themselves).
If you'll be using stock motors for any length of time it'd also be a good idea to swap in a pair of carbon brush packs from a pair of donor FC130 motors. The standard stamped copper brushes you get in FA130 motors (and clones) don't last too long at higher voltages and/or current draws.