Believe me, you'll know when she's in heat. She'll meow her head off. No, meow is too "soft" a word --- she'll scream her head off night and day for as long as a few weeks, or until she gets pregnant. She'll scritch her butt up in the air. It pure heck on earth being around a cat in heat.

They can go into heat as young as 4 or 5 months and it can take til nearly a year old before their first heat. Every cat is different. All mine got spoayed before they could even hope to experience their first heat.