Do fillings last forever ?

"Do fillings last forever" is difficult to answer since there are so many factors at play. There have been fillings that last over 40 years while others fail within 1 to 2 years. The factors can include:

Type of material (silver amalgam vs. Resin composite).
Location in the mouth.
Oral hygiene.
Sugar intake.
Occlusion (how the teeth come together).

So, while occasionally a filling can last a lifetime, that is not the norm. Just like nearly all other medical appliances and devices, they can and do wear out. It is a myth that all fillings will last forever.