Maybe around10 mph or greater, cross wind. One day, shooting 385 GPs, in a steady heavy cross wind, I was getting group centers shifted by 4.5" at 50 yards. The groups were good, about 1.5", with open sights, owing to steadiness of wind. At one hundred yards, the POIs would have been totally off target holder. After returning from the range, i calculated a cross wind of 29 mph.
Windage effects on grouping is a function of wind speed fluctuation. Let's say you point at .5 MOA, thats very good pointing in my book, and lets also say the wind fluctuates by more than 1 mph at the times the shots were taken. Well in these kinds of conditions, with this kind of pointing, assuming perfect bullet stability,it is impossible to get a 1" grouping except by pointing worse than .5 MOA and accidentally getting the group.

Sounds kind of harsh but it is true. One can improve grouping in fluctating wind conditions by choosing heavier, ballistically superior bullets. These kinds of bullets simply are more resistant to wind deflection. Basically if one calculates a ratio of weight per bullet (total) surface area, he will have a number which relates the bullets resistance to wind deflection. I would consider the most wind resistant bullet available to be a 620 Bullshop. The 270 sabertooth, like the 270 BB ballet are on the lower end of the scale having little resistance to wind deflection. PRB is even worse. It is good to know how your rifle shoots in wind so that it can be strategized into your hunting.