Generally speaking the length of the party and the number of guests depends upon the age of the birthday child.
For preschool children the party should last no more than 1 hour.
The party duration for children ages 5 to 6 years old should run from 1.5 hours to 2 hours.
For children ages seven to eight years old, the party should not exceed 2.5 hours.
Finally, the party should not exceed 3 hours for children eight years and over.
Regarding the number of guests, the rule of thumb says that you should have about the same number of guests as the age of the birthday child. Of course there are exceptions. Some children are very capable of handling more guests than their age would suggest therefore it is perfectly okay to invite a few more guests than the child's age. The primary concern is to not overwhelm the birthday child.
For preschool children the party should last no more than 1 hour.
The party duration for children ages 5 to 6 years old should run from 1.5 hours to 2 hours.
For children ages seven to eight years old, the party should not exceed 2.5 hours.
Finally, the party should not exceed 3 hours for children eight years and over.
Regarding the number of guests, the rule of thumb says that you should have about the same number of guests as the age of the birthday child. Of course there are exceptions. Some children are very capable of handling more guests than their age would suggest therefore it is perfectly okay to invite a few more guests than the child's age. The primary concern is to not overwhelm the birthday child.