The quarantine will last until the last sick horse has been well for at least 2 weeks, or for 30 days without a sick horse......whichever the vets in that area recommend.
I went thru the strangles ordeal last summer. It went thru my whole herd, and I was under quarantine for 4 1/2 months. Bleach became my friend. I used it on everything, everyday!!! It was a lot of extra work, and a very stressful time, but all horses and myself got thru it.