The force that is created by wind speed changes as the square of the speed increase in other words double the speed quadruple the force it your vehicle will experience. It's just the same as the drag due to air resistance your vehicle experiences as it goes faster.
As others have said, there is no simple single figure you can use, because relative direction wind will change the effect and severity it has.
Most outfits w ell matched outfits will tow at 60mph, but add in a head on wind of 40mph and who knows it might well become unstable, but a 60mph wind from the side and I suspect it would be very unsettled. And of course teh big problem is most roads will take you in all sorts of relative directions to the wind, so it really is an unknown.
As Ray suggests its a question of trying it and when you find a wind speed that begins to make the outfit wander or tug at the car in an unsettling way, ease off and stop until the wind drops.