It all depends what storms are going on nearby, but it's rare that it will rain all day every day. When we were there last year, over Labor Day, it rained every single day for 2 weeks- in Orlando and then in Key West. It didn't stop us from having a great time though! The Orlando portion there was plenty of sunshine, and luckily a lot of times the rain was at night, or it was the standard midday downpour. The great thing about Disney is most of the rides are inside, so rarely close. 7DMT, barnstormer, etc closed for a little bit maybe 1 or 2 days, but opened back up shortly after.
Bring or buy ponchos! You'll have fun no matter what