Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
No follow-up is usually needed for a small sore that heals within one to two weeks and does not keep returning.
Schedule a routine appointment with a dentist or healthcare professional if sores recur often, interfere with daily activities, or seem linked to a sharp tooth, braces, dentures, or another dental problem.
A dental professional may be able to smooth a rough surface or adjust an appliance.
The article also says
An ulcer that remains present after about two weeks should be examined, even if it is not very painful.
Seek earlier care for a rapidly enlarging ulcer, multiple severe ulcers, fever, weight loss, or symptoms involving the eyes, skin, joints, digestive tract, or genital area.
People receiving chemotherapy, immune-suppressing treatment, or care for a condition that weakens immunity should contact their treatment team for guidance about new mouth sores.
Their evaluation and infection risk may differ from those of otherwise healthy people.
Every sign above is quoted from the sofidex article on Canker sores. If you are not sure, err upward — it is always reasonable to be checked.