Hi Sharon! It’s hard to mess up an oxymel – if it’s too acidic for you, just stir in more honey. (And vice versa – could add more vinegar for a tangier oxymel.)
I’ve actually never had an oxymel go bad before I used it up, and only once had a vinegar spoil – I had used fresh herbs, they didn’t stay submerged, AND I forgot to strain them out for months.
In that case, there was a clearly unpleasant smell, some black growth in the vinegar, and a slimy look to the leaves.
If you’re worried about shelf life when you’re just starting to dabble in oxymels, one thing you can do is use dried herbs and refrigerate your oxymel and try to use it up within 5 or 6 months. That way you know it’s good and fresh for you! :)