It totally depends on your method of making it. If you add more sugar you will ensure better durability. If you can your jam and make sure the jars are sterilized then you prolong the shelf life of the jam. The water bath is a debatable question since many people don’t use this step but if you do process the jars in a water bath it will take the safety one level forward. So, if you follow all the safety steps your jam should definitely last you at least a year and potentially a few years. However, the flavor will be best in the first 12-18 months!