If rheumatic fever was very very mild, what's the likelihood of relapse if i don't continue long term antibiotics?

Hard to comment: Without a full review of your history, records, exam & understanding of how the diagnosis was made it is hard to comment. RF is an immune mediated disease and in theory any new strep A infection could trigger a relapse & more damage. However, there are also proponents of discontinuing long term treatment in adulthood. It is best to discuss your options with your docs.