My idea of how long a party lasts best may differ from your own. I have found the general guidelines below to work very well for my children as they have grown up:

Ages 3-6: A good party length for this age group is 3 hours.
Let’s walk through a typical party:
Guests arrive around 4:00 pm.
The first half hour is spent with the guests arriving. There is general excitement in the air, and you guide the kids as they arrive into one area for unstructured socializing (with supervision of course) while you collect coats and gifts etc.
4:30pm - 5:00pm The Game or Activity you have planned. A half-hour is generally a good time frame for the structured playtime at this age
5:00pm – 5:15pm Clean Up
5:15pm – 5:45pm Eating dinner
5:45pm – 6:15pm Cake, singing Happy Birthday, then the gifts
6:15pm - 6:30pm Clean Up
6:30pm - 7:00pm – Quiet time activity. At 7’ish, guests are picked up for home time. Hand out goodie bags as guests head out the door with thank-you’s to parents and to the child. Make sure your child is at the door participating so they learn by example how to be good hosts.

Ages 7-10: A good party length for this age group is 4 hours. If you have the party on a weekend, you could stretch it out a bit longer without risk of it “going wild”. Your structured activity could last for up to an hour, or you may have a couple of games planned instead of just one.

Ages 10-13: Overnight (sleepover) parties are popular, especially with girls. Start the party with unstructured social time, snacks and then dinner, and plan your activities afterward. Girls at this age will still enjoy a craft etc. and are open to these activities. Boys at this age are boisterous and will benefit from some physical play, so plan an outlet to this energy.

Of course you will not want to “watch the clock” and switch activities like a robot forced to follow a tight and rigid schedule. However, planning out a basic structure for the party will prove very helpful to keep things focussed and to make sure you don’t run out of time to fit all the essential elements into the party.