IMO it will vary sepending on make of freewheel and the quality of materials used. The White Industries freewheels, particularly the special trials version, are probably the strongest as they use 3 or more ratchet arms internally.
The calculation provided by Doohickie is probably the best estimate you will find.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
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