Much depends upon how you care for it. If you aren't religious about powering down before adding or removing peripherals, you will eventually destroy some custom chips. This is fairly serious if your machine uses newer surface moount chips.
Keyboards and mice have a finite life. I try to press gently on the keys (I'm not a fast typer), it seems to help.
Hard drives, I like to have multiple identical HDs (and partitions) and back them all up to each other.
Plastic parts seem to turn yellow if exposed to clensers (409, Windex, Fantastic, etc), sunlight, sticker glue, and smokers. Oh, and fog machines, grin.