Cats stay in and out of heat for most of the spring through fall months.
Indoor cats or cats in warm climates can cycle all year long. If you want kittens later, you can get temporary relief from her howling by inducing ovulation using a Q-tip to simulate mating. Unspayed cats come in and out of heat every two weeks or so for at least three seasons of the year. These heat cycles can notably alter your cat's behavior. I've seen people rush into the veterinary emergency clinic with their young female cat thinking she was very ill or in pain, when she was merely in heat.