Your commitment on any plan is only one month at a time. You can cancel at the end of any monthly anniversary.
Even if you pay quarterly or annually, you can get a refund of the unused months if necessary.
In most cases you are committed for a whole month at a time, although some companies may refund part of a month if requested.
What an insurance company is entitled to is called "earned premium". You owe for the period for which the insurance company has liability.
Until you give notice of cancellation, you owe earned premium until you cancel. Cancellation should be made in writing, unless they take it over the phone and you get a written acknowledgement of the cancellation.