Keeps a long time, years and years, especially if you gave it the full canning regime of preheating the jars and lids, etc. Lacking niter knowledge, I'm pretty sure we even did the boiling water bath on them (old style, few inches of water and the lid on the pot not the new style fully submerged stuff they are promoting now.) Not that I recall getting any niter from that...
I've had syrup grow a bit of mold, but it wasn't the stuff canned (with proper care) in mason jars, it was the stuff rather more casually stuffed into a gallon glass cider jug at the end of the season when I had filled every mason jar we had, bought more, filled them, and opted not to buy any more, but still had syrup. Removed the mold, boiled the syrup, even that was fine.
Sugar keeps even better, and would have been a better thing for me to have done with my excess that year.
Two turkey pans on cinderblocks in the 1970's
4x5 no-baffle stainless pan, built sugarhouse ~1980 - buckets and snowshoes. 17 gallons in best year. Went off to college, nothing for 25+ years.
Thinking about getting going again in new location, in a small way. •• Cats, Coffee, Chocolate...Vices to Live By.