The length of your Halloween party should depend on the guests. If you have children over for a kid's party, 2-3 hours is plenty of time for a few fun games and some sugary sweets. When you have a mixed-age party with families or a high school group, shoot for 3-4 hours like a standard party. The adults will help keep the kids in line and appreciate having more time to socialize. Finally, a more extended party is in order if you are having a party for college kids or those up to around 30 years old. Start anytime after dusk, and plan to go until 12-2 am with more music and drinks.