Question: If I'm making Homemade Jams/Preserves, how long will they last?

Answer: 
"Use within a year" is a good guideline. There are exceptions. Lingonberry jam done right is easily good for a couple of years.
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!