Hi Steve,
We use out injectors every day and have yet to rebuild one. Once in a blue moon I put some worm oil in them and wipe them out. I did replace the o-rings in one just so I could provide help to someone if they needed it. I think they are overall pretty low maintenance (at least that has been my experience). I think I would treat them like the tires on your car, keep an eye on them, check them for signs of damage, provide maintenance as needed.
Jason