Our class reunions have always been spaced five years apart. My husband's (he went to a different high school) also have been five years apart. But know of some who have them closer together and some places are farther apart. I think it is more or less based on what has been the routine for an area. I think that maybe class size has something to do with how often a reunion is held. My husband and I both went to high schools that were small (my class was the largest graduating class in the history of my high school, we had 72 people graduate). But our son graduated from a school in St. Louis that had about 400 people graduate. They only have reunions every 15 years.