The time that a female cat can stay in heat will vary as some female cats only stay in heat for one day while others stay in heat for a week and sometimes longer.
I had a cat stay in heat for 2 weeks once but most cats are in heat for only as long as 7 days.
Although the complete estrus cycle of a cat is 1-3 weeks and during the time your cat is in heat you should isolate your cat from any other cats and keep your cat warm and distracted.
Also you can use Pheromone Sprays and a Feliway Diffuser to help your cat in heat.