As with most things in long range shooting, there are a lot of variables in determining how much wind is too much. With my muzzleloader, my main hunting gun, I am comfortable up to my 400 yd max range with winds up to 10 mph as long as it's pretty constant. Over 10 mph, I pretty much cut the max range shot I will take in half.
I consider the 300WM I have worked up for elk a 600 yd gun under most conditions...myabe 800 yd if it was really calm. Now most guys on here would laugh at that, a 300WM is good past 600 yds. Well, it can be, but I don't have a wind meter, and I prefer not to need a pocket PC with exbal. My gun and a laser rangefinder are enough to feel very comfortable to 600, but if the wind gets heavy enough to worry about I just won't be shooting.
I have lurked here quite awhile and really admire some of the shooters here and what they can do. The gear, preparation, and knowledge amaze me and I'm here to learn from it. Occasionally I see something like "I guessed the wind at 950 yds and shot the animal in the butt, and after 3 more shots I finally put one in the vitals". This makes me cringe because that's not what long range hunting is about. It's about extending your abilities to long range and staying in them.
Could I get the drop right with my 300WM at 1000 yds, guess within 3-4 feet (minute of elk) on windage under most any wind and at least hit the animal every time? Probably, I'm decent with the gun in the wind. But that's not what LRH is about so I limit myself to what I am personally comfortable making every shot with. I've NEVER failed to make a first (and only) shot vitals hit on a deer with my muzzleloader at ranges to 338 yds, so it seems to be working for me.
Practice. A LOT. Figure the range you can reliably hit the target at time and time again under field conditions. That's your max wind and range.