They can hold on for two-three days. If you're asking in the context of a cat who is straining to go to the toilet or only urinating small amounts you need to go to the vet ASAP as this can be a life threatening condition called Cystitis where crystals form in the bladder. This can kill a cat as the bladder fills up and the crystals get bigger the likely hood of the bladder bursting without medical treatment is high. Hope I helped.
