Normally, a cat's hiccups will stop within a day at most. However, if a cat hiccups frequently and often, it's worth considering whether the cat has eaten too much or has a foreign object in its throat. Furthermore, it's important to note that if a cat's hiccups persist for several days, it could be a sign of a brain tumor or a problem with its nervous system, and it's best to take the cat to the vet for a checkup. 
What to do if your cat hiccups frequently:
1. Ensure there is an ample supply of food and water. Provide your cat with plenty of food and water, and ensure a quiet space. Do not try to force your cat to eat or drink. If your cat prefers running water, you can try using a water fountain.
2. Change your cat's habit of overeating. You can place the food in a higher place that is not easily accessible to your cat, or use a food strainer to slow down the eating speed.
3. Solve the hairball problem. You can try using cat grass or hairball remedy to stimulate the cat's intestinal peristalsis and help the cat expel the hairball.
4. Take your cat to the vet. If your cat hiccups frequently but you can't find the cause, or if the hiccups last for more than a day, please take it to the vet.
