My daughter loves to wear her princess dresses. This past trip we were at WDW in December, so it was cooler. But our usual time to visit is in September. And even with 95 degree weather and 90% humdity, she will wear those stinkin princess dresses all day long. I feel the CM's make a bigger deal of the girls when they are all made up. Plus during character meets, she's had Stitch, Mickey, Pluto, Chip and Dale plus the Beast bow to her dressed in her princess garb. She loves it.
She wears it from the time we leave the hotel room until we come back. But not all kids are like my DD so you may want to take a change of clothes in case your DD gets tired of her dress.