In the two years I worked there, I never saw a weekday crowd in May that I would describe as anything more than moderate. Even weekends aren't that bad in May. Sure, there might be a few long-ish lines, but nothing is going to be over an hour on a weekday in May, unless something really weird happens.
I went in on a Sunday in May 2008 (off day) and was able to ride 39 separate rides, that's how dead it was. And I probably could have ridden another 4 or 5 if they were open (Maxair was still down, Sky Ride was down for wind, TTD and DD went down for rain, Wildcat was down because of the lift hill incident that year). Granted, the wet-ish weather drove some of the crowd away, but even before it rained it wasn't crowded at all.