I have a 16-inch Dob with a cloth light shield over truss tubes and it's pretty rock steady. When the winds are annoying enough to knock the cloth into the light path, well...
As for bobbing the scope around, the majority of my viewing is done at 102X with an 82 degree EP, so it can take a bit of bobbing. However, the real factor is the temperature, for one, and how much it blows my charts around on the table. Also, if I'm looking very high up and the wind moves the scope when I let go of it...
As for wind speed, I'm not really sure.
There are occasional gusts = no big deal.
Steady breeze = borderline annoying.
Steady wind = annoying verging on quitting.
Strong gusts settling to a stiff breeze = pack it up.
Miles per hour? Hard to say. I've never measured it. The Clear Sky Clock may say one thing but the actual site where I usually observe is eight miles away in an open area, whereas I'm a bit sheltered. That might or might not make a difference with dampening or venturi effect. Therefore, exact MPH which is always a range, say 11-16 MPH or something, is vague as it is.
I'll go with my assessment above and avoid the numbers.
Besides, I'm allergic to math.