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 or swelling of the lips, tongue, or throat after using a medicine or skin product, as these can be signs of a serious allergic reaction.
And for anyone, someone who:
Prompt assessment is appropriate for increasing redness, warmth, swelling, pain, pus, or fever, as these may indicate a skin infection.
And for anyone, someone who:
Arrange a routine appointment if you develop persistent itchy patches or bumps, particularly when relatives have similar skin changes.
An examination is important because eczema, lichen planus, and other skin conditions can look similar.
The article also says
Seek earlier medical advice if:
Itching is disrupting sleep or daily activities
A lesion develops a new lump, ulcer, or repeated bleeding
Treatment causes persistent burning, thinning, or worsening irritation
The diagnosis is uncertain or the skin changes are spreading quickly
Familial cutaneous amyloidosis itself is not usually an emergency.
Every sign above is quoted from the sofidex article on Familial cutaneous amyloidosis. If you are not sure, err upward — it is always reasonable to be checked.