I also think that the dessert party is for those couples that want to have that special viewing area for Illuminations/Fantasmic, but don't want to have a full reception there. As for the timing between reception and dessert party, well that depends on you. Are you having a dj/band and plan on dancing awhile, or are you doing something more on the lines of just dinner? I would imagine that those couples having that are having the dancing and bigger party, the reception will be 4 or 5 hours; therefore would need an earlier wedding time for a dessert party. If you are just doing a nice dinner, you might be able to get away with a later wedding time to have a DP.