Cats typically stay in heat (estrus) for about 4 to 7 days on average, though the length can range from as short as 1 day up to 21 days depending on the individual cat

. If the cat is not mated during this period, she will usually go out of heat for a short time (about 7 days on average, but it can vary from 2 to 19 days) before returning to heat again

. Indoor cats that are not spayed may go into heat every 2 to 3 weeks during the breeding season, with each heat cycle lasting roughly 3 to 5 days

. The heat cycles can continue repeatedly throughout the breeding season, which typically runs from spring to late autumn but can be year-round in warmer climates or indoor environments

. In summary:

• Heat duration: about 4-7 days on average (range 1-21 days)
• Interval between heats if unmated: about 7 days on average (range 2-19 days)
• Frequency: every 2-3 weeks during breeding season if not spayed

Spaying ends the heat cycles altogether. This cycle begins when a cat reaches sexual maturity, usually around 6 months of age but can vary from 4 to 9 months

. Thus, cats in heat usually exhibit this phase for about a week, with some variation, and will cycle repeatedly if not bred or spayed