it depends on a number of things. how much you sweat, the weather (cold, humidity, rain, snow etc) will all affect it.
a thermal lense solves most problums as far as steaming up goes, for most people although a couple of my friends who are very heavy sweaters have masks that still steam up with thermal lenses, and have fans installed in them aswell
bottom line is, yes it will more than likely steam up, but if you are playing on a hot dry day in the summer it probably wont. get a mask with a thermal lense, PM me if your after a dye i3 or pro i3, or pick a lense up on here (use the "wanted" section). if you already have it, upgrade it. the point is it is much more likely to steam up, you dont want that.