How long do cats stay "in heat"?

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How this Happened in the First Place: Baby Girl Grows up
Many people are surprised to find that the feline reproductive system is seasonal. Cats are designed to give birth only during warm months (spring through early fall).
The female cat begins cycling when she has reached 80% of her adult size and when the days are appropriately long. This could be as young as 5 months of age. After age 7 or 8 years, cycles become irregular and there are more complications with birth so it is important to finish a female cat’s breeding career before this time.
Indoor cats are sheltered from the light cycles of the sun and may not cycle as frequently as their outdoor counterparts. On the other hand, breeders who keep their cats completely indoors can manipulate the light cycle of the cattery so as to allow for year round cycling if desired.
Proestrus: This is the time in the cycle when the female begins attracting males but is not yet receptive to their advances. Estrogen is building up and she is preparing for ovulation. The female is extra affectionate at this time, rubbing her head and sticking her rump in the air. She may also urine mark in the house and vocalize loudly and frequently. This period may be as short as 12 hours and as long as two days.
Estrus: The female cat’s behavior continues: rubbing, crying, etc. but the difference here is that the male is accepted when he approaches. This behavior persists approximately 7 days (on the average) and during that time the female’s behavior is often objectionable to the owner. Contrary to previous belief, the act of mating does not shorten the estrus period, although it does substantially lengthen the time before the next estrus period. The female cat is ready to ovulate at this time but will not do so unless a male cat breeds her.
Interestrous Period: This is the period between times of behavioral estrus. If the cat is not bred or is bred and fails to ovulate, this time period lasts 8 to 10 days on the average. This means that the yowling, rubbing, urine marking, and other estrous behaviors continue for about a week, then discontinue for about a week, then begin again, back and forth all spring and summer and into the fall until the cat is either bred, spayed, or perceives the coming of winter. This behavior of the mature female cat is often all the motivation it takes for an owner to get her spayed even if she is completely confined indoors and has no chance of unwanted pregnancy.
Diestrus: If the female is bred and ovulates, she goes into this different reproductive stage. Her ovaries prepare for possible pregnancy and she will stay out of heat for at least 35 to 37 days even if she is not pregnant. If she is pregnant, she will carry her kittens for 64 to 66 days before delivering the litter. If for some reason spaying is desired but must be delayed, and yet the estrous behavior is driving the owner crazy, she may be fooled into thinking she has been bred through manipulation with a rectal thermometer. If you are interested in this procedure, contact your veterinarian as it is not as simple as it may sound but is often effective in providing peace at home. Thanks to Dr. Wendy Brooks for this great summary of information!