Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency help if decreased appetite occurs with:
Arrange same-day medical care if you cannot keep fluids down, pass very little urine, have a very dry mouth with dizziness, or show other signs of significant dehydration.
Also seek same-day advice for worsening weakness, repeated vomiting, or severe pain.
Seek care sooner for difficulty swallowing, ongoing vomiting or diarrhoea, persistent fever, yellow skin or eyes, marked fatigue, or continuing low mood.
Book a routine appointment if the change lasts more than about a week, keeps returning, or causes unplanned weight loss.
The article also says
Children, frail older adults, pregnant people, and anyone with diabetes or another condition affected by food intake may need earlier advice.
If you use insulin or another medicine that lowers blood sugar, follow your sick-day or low-blood-sugar plan and seek help if you cannot eat safely.
Every sign above is quoted from the sofidex article on Decreased appetite. If you are not sure, err upward — it is always reasonable to be checked.