Good quality balsamic vinegar can be rather expensive due to the long process that is required until the final product is obtained. However, it can be stored indefinitely, in a cool and dark place away from heat, as oxygen does not create a problem when the bottle is opened and the vinegar will not deteriorate. After a time there will be some sediment in the bottle, but it is not harmful in any way.
Keep in mind that some cheaper versions of balsamic vinegar contain sulfites. Check the labels carefully if you happen to be allergic to sulfites.
Balsamic Vinegar has a certain amount of acidity. If it is heated up, it becomes sweet as the acidity is boiled out. If you don’t like it sweet you can add it after the cooking is completed.