There's already a reasonably similar thread somewhere round here talking about how old some people's Macs are. The G5 should last you for a VERY long time. Any problems you may have with it will be in the first few weeks which are usually bad components but this is pretty rare and they can be replaced easily under warranty.
Of course, the G5 hasn't been around long enough for an actual or scientific response to your question but going by what other Mac hardware has achieved, you should be absolutely fine with that for another seven or eight years. Just don't hold me to that.
Apple put so much care into crafting the G5 machines. With 9 fans, no parts are working too hard, no parts are stressed. And you are not getting rev. A. The initial kinks have been worked out. The machine should last a very long time.