I've taught a fairly large introductory course, and every exam we'd have maybe 1-5% of the class taking a 1.5 or double length exam in a separate room, depending on the severity of their "disability." I'm not an expert or anything, but all of these kids looked fine for me. I suspect a lot of them got their disability certification while in public schooling at the behest of their parents, who wanted to give them an advantage.