Various online services have different values for maximum year of expiry, when it comes to Credit Cards.
For instance:
What is the reasonable maximum here? Are there any official guidelines?
There is no official guideline as the credit card issuers can choose each when the cards they issue will expire. In fact they have been issuing cards for longer and longer periods of time. If you're trying to determine how far out into the future you should accommodate expiration dates for, err on the safe side and give your customers many years to choose from. That way you future proof your application.
FYI, many credit card issuers do not use the expiration date when determining whether or not to approve a credit card purchase. So if you're worried about an incorrect date being provided the processor will ultimately have the final say on whether the transaction is approved or not so I wouldn't worry about it.
July 2017: Just had an end user with a card that expired almost 50 years from now.