15 years. Composite fillings lasted 12 years on average. Yes, there are extremes on both ends depending on how well people take care of their teeth and how large the fillings are.

The people who only brush and floss before going to the dentist and drink Mountain Dew all day are lucky to get 3 years out of dental work. Those who take good care of their teeth and have a good diet can see things last for a very long time.

But you don't replace fillings just because they are x years old. You replace them if decay is growing around them or the edges of the filling start breaking down, creating a gap for decay, or if they crack.

If the integrity of the seal is broken, decay can develop and hide under the filling. It is better to just replace the filling at that point.

Some guys wait until the wheels are coming off of a tooth before they do anything. Some treat problems while they are small. In my opinion, if you want to keep your teeth healthy for your life, you don't want to go to someone who waits too long to treat things. That means more crowns, root canals, and missing teeth.

Of course you don't want a dentist treating things that are doing just fine.
. . . but waiting until teeth start to hurt is just asking to have root canals and crowns.