How long can vary depending on the hotel.  The larger chains all have a permanent data base for all reservations and all information collected with those reservations-including CC info.  

There ARE consumer protection laws in every that protect you from this sort of thing but nothing is really stopping the hotel from trying to charge your "incidentals" CC for whatever disputed charges may be taken off of the other card.  You would then have to fight the charges on that new card with that CC company.  You are far better off trying to settle this with the hotel.  If you are not getting satisfaction from the billing department it may be time to bring your issue to a higher level of management.

It is irrelevant how long they keep your credit card info, but the answer is years.

You can initiate charge back proceedings with some cards up to 9 months. It depends on the merchant bank.

IF your charge back is granted they would probably not charge your card a second time as they would be in violation of their processing agreement. That does not mean that they will not pursue you to get paid.

If you feel you got charged wrong, feel free to dispute the error with you cc company. If they charge the second card, do the same thing.

This type of stuff happens all the time at hotels. Some times intentionally. Usually the hotel hopes you just go away because you don't have time to dispute.

There is a form at the back of statement that you need to fill out and mail to the cc company. If you get your statement online, you can print it out online. To preserve your rights, make sure you mail the form and not call.

Save and mail copies of all the info you have to the cc company including reservation confirmation, receipts, signed check-in statement with rate indicated and check-out statement if you have one. Also have info on the dates you stayed and rate you should have been charged.

Before going through all this trouble. Keep in mind that hotel rates change frequently especially from weekday to weekend and even daily with larger established hotels. If you did not confirm the rate for the part of the stay that you extended, you most likely got charged the "highest rack rate" available for your room and might be out of luck.

Also don't forget to factor stuff like taxes which in some cities is usually 15-20%, late check-out fees, damages to the room, smoking in a non smoking room, thrashing the room up, room service, gratuity etc.

Hotels usually are required by the card companies like VISA, MC etc to save cc information and transaction info for as long as 2-3 years. State and local law might require them to save that info for longer. The bank that issues your card usually needs you to file a dispute within 60 days after you received your statement.

When you mention "incidentals," you are not clear if you are referring to stuff like restaurant charges, room service or stuff you take out of the stocked refrigerator. In some cases it is difficult to split room service or stuff you pull out of the fridge charges to a separate card because the hotel computer software can only store one card and all charges are processed automatically right away through the same pos system when you check out.

My guess is you probably confused the hotel staff trying to use two cards but it is fixable if you file a dispute. Next time use cash for the "incidentals" if you need to separate the charges out.

As I mentioned, I was oversimplifying the situation. Thanks again, and I will make sure to exercise my due diligence.