It's a single processor, right?
Theoretically, if you are happy with the software on it now and you never upgrade any of it, you should be fine indefinitely however we both know this isn't really feasible nor sensible. I'd say any G5 should be competitive until at least a year after the Power Macs move to Intel and usable for another four or so years.
Of course, it's hard to say since we don't really know how well these things will age and how fast technology will move ahead over the next decade.