Douglas,
I just replaced mine on this haulout. I also had the Spartan unit. It was probably fine, but with all the problems I had with my entire drive train setup I decided to replace EVERYTHING.
I purchased one from Buck Algonquin through a marine store that carries there stuff here in St. Thomas.
You’re asking about a traditional bronze stuffing box or shaft log, right?
It’s one of the few things on a boat that, at least theoretically, doesn’t need to be replaced. Ever.
That’s because the bronze stuffing box suffers no stress or wear. The bronze parts don’t contact any moving parts - they float above the prop shaft, separated from the shaft by the packing.
So absent any unusual mechanical stress (such as when the prop gets wrapped by rope causing the shaft to move eccentrically), thermal stress (eg if the gland is overtightened causing over-much friction between shaft and packing, which is usually shown by the packing changing color), electrolysis, and the like (eg if a shaft is not turned for a long time, organic matter in the seawater can decay and lead to the formation of hydrogen sulfide, which can chemically attack the bronze), the bronze stuffing box should not need replacement.
Inspection for cracks and electrolysis etc is however essential. Just like everything else.
I’ve been on a trawler that was still running her original bronze shaft log after 40 years and a couple of engines. And I doubt that trawler held the record.