It varies by lender. Some say 30 days after your statement posted, which would mean in most cases about a week after the payment due date. Some say 30 days after the payment due date (this is the most generous.) Some say 31 days after you made the charge --hey, whatever happened to the grace period? And some, as RobertEG said, smack you with it the day after the payment is due.
There's late for the CCC, where you get a late fee and maybe rate-jacked, and then there's late that gets reported to the credit bureaus. They're usually not the same date. For instance, your payment was due on the 20th, you paid on the 22nd, you get a late fee and maybe your APR changes to default rates, but it isn't reported to the credit bureaus as a 30-day late.
You can try to figure this out from your T&C (terms and conditions statement), or you can try calling customer service and asking how they calculate it.
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007