I'm the same way, I normally don't reapply it for maybe 3-4 months, but that is only when i'm tinkering, like when I recently went up to arctic silver 5. So far it's been running great.
Unless there is any bennefit of doing it longer, the stuff shouldn't wear out.
Like thermal paste, it will last a long time, I've even seen a pentium machine with it on and it is still used and never reapplied it.
However if it were a thermal pad, some of those do wear out, especially at a pretty hot tempature. Have heard of the actually melting. However I wouldn't use thermal pads. Thermal grease I could go for on someone elses computer if they wanted to repair it

I don't like to waste arctic silver on other people, since i do use a good ammount of it for myself.
If anyone wants better, then they can go spend 10 dollars on a little tube of it.