How many months upon request of cancellation of my homeowners policy LEGALLY can insurance company back date my cancellation date?

Based on your question it appears you requested a back dated cancellation of your homeowner policy.  The insurance company asks that any change, or cancellation, be requested in a timely manner.  However, if you fail to do so, the company has the responsibility to work with you on a backdated request.  If the request is over 60 days, the company can request paperwork evidencing the date of sale of your home.  This would be a copy of the sales contract or similar paperwork showing the date, since you no longer had any insurable interest in the property as of that date.   You should speak with your agent or company if they are refusing to backdate or if you have no paperwork proving the date of sale.