It all depends!
Let's say you run 6v, direct drive. A teeny prop won't load the motor much, and it will spin at 8500 rpm, cavitating all the way. A huge prop will load it down, making it run slow AND pulling lots of amps. How slow? I have no idea. Experiment- make a rig with the motor, shaft and prop, and stick it in a bucket... if the motor gets too hot to hold, you are mismatched! Even better if you can rig a tachometer somehow (I build my own with a photo-interupter and read it on an oscilloscope).
If you run on 12v, you will need a gearbox... again, I can't predict how fast the motor will run then... but if runs at 75% (12,750 rpm), you'll want a 1.8 reduction.