Like all things with insurance there is no definitive answer about how much money you will get back when an insurance policy is cancelled. There are things called short-rate, pro-rata, and minimum earned premium that can all impact how much money you get back if your insurance policies are cancelled. Each of these things might be trumped by the financial penalty of not cancelling your policy if it is not providing adequate coverage and you have a loss.
This video explains it all in a way so that you can be prepared if you need to cancel your policy.
MattCorc 