Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services for severe breathing difficulty, severe or persistent chest pain, collapse, or a person who is unresponsive.
These symptoms can have several causes, but they require immediate assessment.
Get urgent medical help for fainting, particularly during exercise, or for a sustained fast or irregular heartbeat with dizziness.
Routine surveillance appointments are different from urgent care.
Do not wait for the next planned review if significant new heart symptoms develop.
Arrange a routine medical appointment if you notice possible features of the condition, especially many lentigines together with short stature, hearing problems, heart findings, or a family history.
This does not confirm the diagnosis, but it may justify assessment.
The article also says
If you already have the condition, seek earlier advice for:
New breathlessness, reduced stamina, or frequent palpitations
Dizziness or near-fainting
A change in hearing
Concerns about a child's speech, learning, growth, or puberty
A skin spot that bleeds, changes rapidly, or looks unlike the others
Every sign above is quoted from the sofidex article on Multiple lentigines syndrome. If you are not sure, err upward — it is always reasonable to be checked.