It is easy to assume a cat that turns up its nose is simply being a cat, and to give it a day or two to come round. With cats that assumption is the dangerous one, because a cat not eating is not on the same timescale as a dog skipping breakfast.
A cat's clock runs faster
When a cat stops taking in food, its body begins moving stored fat to the liver for energy, and a cat's liver handles that flood poorly. In an animal that was carrying extra weight the process can overwhelm the organ within days, a serious condition your vet will recognise and treat but which is far easier to prevent than to reverse. This is not a rare worst case; it is the reason vets treat feline appetite loss with urgency.
How long is too long
A cat that eats nothing for a day, or that eats markedly less for a few days running, has crossed from fussy into worth-a-call. Bring the call forward if the cat is also hiding, drooling, vomiting, or visibly losing weight. There is no home remedy to reach for first here — the point of acting early is that early is when it is simplest to fix.
Plenty of causes, one next step
Appetite loss sits on top of almost anything — dental pain, nausea, stress from a change at home, or a genuine illness — and the range of possibilities is exactly why guessing is the wrong move. A cat off its food is a signal to have looked at, not a puzzle to solve at home, and the vet is the one who can tell a sulk from a symptom.
How long can a cat safely go without eating?
Not long — a cat eating nothing for around a day, or much less than usual for a few days, warrants a vet call. Cats risk a serious liver condition when they stop eating, so this is treated with more urgency than in dogs.
My cat skipped one meal — should I worry?
One missed meal in an otherwise bright, normal cat is usually fine to watch briefly. Refusing food across a day, or eating little for several days, or any illness alongside it, is a reason to ring the vet rather than wait.
Why is a cat not eating more serious than a dog not eating?
A cat's liver copes badly with the surge of fat the body releases when it stops eating, and in overweight cats this can become dangerous within days. Dogs tolerate a short fast far better.