If you notify your insurance company that the property will be vacant for an extended period of time (more than 3-6 months), yes they will most likely cancel your policy. If you don't tell them, your policy will stay in force as it's not like they often cruise the streets looking for vacant homes.

Under the insurance regs in most states I know of, If you were to incur a loss after an extended vacancy, they would still have to cover the loss if you did your due diligence to secure the property and were actively seeking to sell, remodel for purposes of occupancy, or rent it.

All of my rentals have monitored alarms for this reason -- even the low end quadplex. I keep a log of when I go by to check on vacant properties. I also keep copies of flyers, newspaper ads, or anything else I do in efforts to keep the place rented.