Patch testing is widely recommended and widely done wrong. The timing that makes it useful follows from how the two different reaction types actually work.
There are two distinct kinds of reaction
Irritant reactions occur when a substance directly damages the skin surface, and they can affect anyone given sufficient concentration or exposure.
Allergic reactions involve the immune system recognising a specific substance, and they occur only in people who have previously become sensitised to it.
The two look similar once visible but follow entirely different timelines, which is the reason a short test misses one of them.
Allergic responses are delayed by design
The relevant immune process involves cells identifying the substance, travelling to lymph nodes and returning to the site, which takes considerable time.
The visible reaction therefore typically appears a day or two after exposure rather than within minutes, and it can build for several days after that.
A test read after twenty minutes captures only immediate irritation, and a clear result at that point says nothing about allergic potential.
Sensitisation requires prior exposure
A person cannot react allergically to something their immune system has never encountered, so the first exposure to an ingredient often passes without incident.
Sensitisation can develop after one exposure or after years of uneventful use, which is why a long-used product can suddenly start causing problems.
This also means a single negative patch test does not guarantee indefinite tolerance, though it remains far better than no test at all.
Where and how the test is applied
The inner forearm is commonly used, though skin there is less reactive than facial skin, so a test result understates what the face might do.
Behind the ear or along the jawline is closer in character to facial skin and gives a more representative result for face products.
The product should be applied as it would normally be used and left in place, with the area kept dry and unwashed for the observation period.
What the test cannot settle
Reactions that depend on sunlight only appear when the treated area is exposed to light, so a covered patch test misses them entirely.
Cumulative irritation from repeated use over weeks is also invisible in a single application, which affects actives such as acids and retinoids in particular.
Anyone with a history of significant reactions should have formal testing arranged through a clinician, since the panels used there identify specific allergens rather than whole products.