Your backers are billed right away the first time they back an artist, and for any other artists they back on that day.
Thereafter, we charge each backer once per month, for all of their subscriptions. This reduces the credit card fees artists pay.
When a backer supports you after their rebilling date, the amount is pro-rated. For example: if, after backing Alice for $5/mo on December 5th, Bob backs Mary for $10/mo on December 27th, he'll be charged $2.14 right away, and that's the amount that Mary will receive for that month minus fees. The subsequent month and in all the months that follow for as long as Bob remains a backer, Mary will receive the full $10.