Depends on where one is located and the weather.
Breeding season in the Northern hemisphere is during the months of late winter and early spring (March-September). Indoor-cats or cats living in warm climates can go into heat and become pregnant more than once during the breeding season.
When in heat, the cat may display several signs. These may vary between breeds and even between individual cats. Signs include the famous calling (loud meowing), increased appetite and restlessness. Many cats begin to roll on the floor and demand to be petted. Some cats may begin to spray urine around the house.
If the cat is not mated and does not become pregnant, she can go into heat repeatedly every 2-3 weeks. This means that if your cat goes into heat - you can't simply keep her locked up at home, waiting for it to go away. Unless you spay the cat, she will keep going into heat practically every month.