5 to 7 days on average once it starts. I get my cats fixed as soon as possible for this reason.
Remember when you were young and friskie?
Listen to Bob Barker... "Help control the pet population... have your pets spayed or neutered"
I have been there. It sounds trivial but I know how annoying it can be. They make a LOT of noise. A couple of weeks to a month at least. Get her fixed.
Squirt bottle, spray her with water. She'll eventually get the idea.
lol I feel for ya!!
Throw he outside for a while at night. Then she won't be in heat.