Depending on how clean your records, how often you clean the stylus, and the care you take in cartridge set-up and alignment, you could get as much as 1500-2000 hours out of the 17D3.
When the stylus is worn and in need of replacement, the most cost-effective option is to have it re-tipped by SoundSmith. Dynavector does have a re-tipping program, but it is much more expensive than SS, and what you get is someone else's re-tipped trade-in (instead of your re-tipped cart). The 17D3 already has a diamond cantilever, so if it's still in good shape, you should not need to replace it. I'm not sure that changing to a ruby cantilever would be much of an upgrade over the stock diamond model anyway.