How long is a insurance company required to give before they cancel your insurance?

My insurance called me yesterday and informed me that they would be cancelling my insurance because I have rented my house to a group of students. They gave me two weeks to find new insurance before they cut me off. I'm having trouble finding someone who will insure us. If they cut my insurance, my bank will cancel my mortgage, and I guess that means I'd lose the house. Is this insurance company allowed to drop me in this manner. When we got the insurance two months ago there was nobody living in the house, but it was rental insurance. We never lied or anything, we were completely honest through the whole process.