how often does iTunes bill purchases to credit cards

When you make the purchase. iTunes will attempt to bill immediately entirely.

If it bills in increments there is no set time interval for it to try.

Should have enough credit on your payment method to cover the purchase at the moment of purchase regardless.

I find that it tends to wait until you have a couple of items before claiming a payment, there doesn't seem to be a strict pattern to it, although since its automated there must be.