We made plum jam in 2013 and still have plenty left. I wonder if they are still safe. We canned them in re-used commercial jam jars with "pop" lids, so we'll at least be able to know if the seal has been broken as we open them. Ingredients were just plums, sugar, and pectin. Thanks!
Food storage instructions usually recommend that home preserved foods be consumed within a year- and that jams or jellies in less time than that for optimal quality. If allowed to remain in storage for longer periods of time, the quality deteriorates; colors fade or brown, jams and jellies change texture, flavors weaken.
However, the contents of the jars should remain safe indefinitely if they were properly preserved so that all pathogens were destroyed when the products were canned, the jars were stored at moderate temperatures and the lids remain sealed. After the first year or so it is up to you to decide if they are good enough quality to use. They may not be very appetizing. Check each jar carefully for any signs of spoilage. In jams look for molds or fermentation.
I would caution you about re-using commercial jars and lids for canning. Jams are a low risk product, but other foods would have a greater risk if not properly processed and sealed. Reused lids aren’t always reliable.