How long does it take for Apple to bill me for purchases on iTunes?

Solution 1:

Per the iTunes Store — Terms and Conditions

If a credit card or your PayPal account is being used for a transaction, Apple may obtain preapproval for an amount up to the amount of the order. Billing occurs at the time of or shortly after your transaction. If you are using 1-Click purchasing or your PayPal account, your order may be authorized and billed in increments during one purchasing session, so it may appear as multiple orders on your statement. If an iTunes Card, iTunes Store Gift Certificate, or Allowance Account is used for a transaction, the amount is deducted at the time of your transaction. When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.

Solution 2:

From my experience and from checking the last couple of months of receipts my average is 24-72 hours with the longest being just under 2 weeks.