If I'm making Homemade Jams/Preserves, how long will they last?
"Use within a year" is a good guideline. There are exceptions. Lingonberry jam done right is easily good for a couple of years.
All home-canned food should be used within a year. This assumes you follow the strict sterilization regimen required by jarring/canning at home. You should also store them in a cool, dark, dry place between 50-70°F.
Over time changes in color, flavor, and texture is inevitable. This will result in a degradation of quality in the product, but as long as the seal is in tact and there are no visible signs of mold or yeast the jam should be safe to eat.
Another thing to note is that lighter colored jams tend to darken faster than others and thus may not look appealing over the course of a full year. They are still "safe" though.
The Good Eats jam episode suggests 2-3 weeks if you don't actually preserve the jam in a home-canning kind of way and about a year if you do.
There's also freezer jams, which can easily last a few years in a deep-freeze without losing much quality. They're well sealed, so there's little moisture loss / freezer burn, there's little loss of quality aside from the initial freezing, and you don't have to worry about mold or other organism growing in the freezer.
Home made Jam made with half Fruit and half Sugar will last almost indefinitely... it's additives that spoil food, placed into in a sterile jam jar it could be there many years and still as good as the one eaten in the first week, Ask your granny not a jobsworth! 50/50 nothing else!