My guess is that it leaks mercury for as long as it's in. When they are being put in and when they are being taken out is when the most mercury is released into the body (hence need for the safeties in removal). Takes about 24 hours for the filling to set properly, so there is a lot of mercury released when they are put in.

As to how long before they need replacing, I'm not sure. Some say 10 years, but most of mine have been there for 20 and I would reckon that several have some life yet in them (as fillings). Others have worn away.