Depends on how well looked after the machine is. Because the 620 is a 4 year old model i dont imagine anyone would have had one for longer then that and therefore it is impossible to be able to tell how long it will last.
The real question is, does it still function well, does it do what you need it to do? My main machine at home is a 5 year old Dell Dim 4500. I've added RAM, larger hard drive, and DVD writer to it, and it still suffices for what I need it to do.
Would I like to have an up to date, fast Core Duo machine? Sure. Do I need it? No.
Then again, I'm driving a 15 year old Toyota pickup.