Filling leak when going in and continually until all the mercury has gone out, this is the point when the filling falls out or crumbles out. Usually takes a long time. I have fillings going on 20 years old that are still hard as a rock. They do recommend replacing them every 7 years, however, everytime you remove and reinstert amalgam filling you get larger doses of mercury. So best to either remove them for somthing else or leave them until you can put something else in. In other words, don't waste time and money removing an older filling only to replace it with a new metal one. As far as research shows, they leak the same amount.