Okay. How much do you weigh? 200 lb.? Maybe a heavy rider is twice that? Apply 400 lb. through your drivetrain, applying the various mechanical advantages (crank arm > chain ring > cog) and that should give you a pretty good idea. I have a 46/18, not sure about the crank length, but let's say it's a 170 mm.

So, force is applied at 170 mm = 6.7 inches

Torque applied about the crank is 6.7 x 400 = 2,680 in-lb (= 223 ft-lb)

With an 46/18 gear ration, the torque at the rear axle is 18/46 x 223 = 87.4 ft-lb.

So the torque applied at the rear axle by a 400 lb. rider putting all his weight on one crank through a 46/18 gear train is 87.4 ft-lb. That gives you a factor of 2 over a 200 lb. rider. If you're looking for a rating for the torque on the freewheel, it's probably somewhere around there, like maybe 100 ft-lb. I suspect the actual rating is a little higher than that.