Yes, graves are reused after 100 years. This is because the average lifespan of a human is around 70-80 years and after 100 years, most people would have died and their graves would be available for reuse. There are some exceptions to this rule though, such as in cases where someone is buried with a specific request that their grave not be reused or in cases where thegrave is part of a family plot that will be passed down through generations.