Its just that most fillings only last a certain amount of years before having to change them or before they wear out and wandering whats the life of the silver fillings and how offen do dentists recommend to replace them?

That is like asking how often a car needs to be replaced. It depends how well it is holding up. Judging from what my dentist said about my fillings, the largest amalgams, especially the ones that were discolored seemed to typically be the ones that he said were most in need of replacement. My filings are mostly 20-35 years old. I believe the ADA lists something like 10 years for the recommended time after which amalgam fillings should be replaced. That seems to be an extremely conservative estimate, and perhaps around 20 years might be a better guideline if they aren't showing signs of deterioration. Of course if you have many of them and feel you may be showing signs of mercury toxicity then the answer would be they all should be removed ASAP.

It's hard because decay can still be going on underneath them and usually it's not realised until it's bad enough to where you start getting pain.

Composites are another one for that.