So, in an average month, how many 'days' pay does one earn?
If you make yourself available every day of the month, you'll get paid for every day of the month. As an ad hoc contractor, you have a great deal of control over your availability.
I guess it depends on your personal situation and where you are based. If you live in your base city and make yourself available every day of the month, you'll be paid for every day of the month, even "off" days. I have never heard of anyone being forced into a "no pay" status if they made themselves available to fly.