I think every time I've gone to WDW it has rained. But the rain would last anywhere from 15 minutes to a few hours (in some cases). It's hot down there and the atmosphere is unstable, so just expect some rain pretty much every day. You get used to it.
And because it's so darn hot, even if you get drenched for a while, you'll dry off fairly quickly when the sun comes out. P.S.: Forget about any hairdo when you're down there--between the muggies and occasional daily rain showers, any "style" doesn't stand a chance. But WDW is vacation playland, and I could care less how I look when I'm down there. I just want to go on the rides and eat ice cream.