Ideally, the sleepover would start just before dinner time. If kids are used to eating around 5:30 p.m., invite guests over at 4:30 p.m. That leaves enough time for pizza, games, and a movie before bedtime.
For older kids and teenagers, you might want to push the start time back a bit, maybe to around 6:30 p.m. or 7 p.m.