Typically, a birthday party for school age children is between two and three hours. This provides enough time for the kids to play, eat, have cake, and enjoy some entertainment. If you’re throwing a party for toddlers, an hour ½ works well, two hours tops. If the party’s for a one-year-old, usually, this means it’s lots of family.
You can gauge how long you think is best based on the typical length of your family celebrations. For example, if you have relatives that love to gather and mingle, you might end up with a three hour event. Otherwise, sticking to the hour and a half might be a better option.