Based on my experience, this is how long a kid’s birthday party should last: 1  One hour for kids under age three. 2  An hour-and-a-half to two hours for a pre-school-age (four to six years old) birthday party. 3  Two to three hours maximum for older kids should be perfect. Small kids have short attentions and any more is just hard on everyone. 2  An hour-and-a-half to two hours for a pre-school-age (four to six years old) birthday party. 3  This gives plenty of time for games, snacks, and cake. 4  Two to three hours maximum for older kids should be perfect.