When a cat is in heat for the first time the cat is usually in heat for a period of 10 days although sometimes the cat may only be in heat for 7 days or even up to 3 weeks.
After the first heat cycle then the cat is usually in heat for a less amount of time.
Usually the cats next heat cycles last from 1 to 3 days.
When your cat is in heat isolate the cat away from any other male cats and keep your cats litter box clean, give the cat something warm to sit on and sometimes you can calm a cat down in heat using catnip but only if the catnip calms your cat and doesn't make the cat crazy.