Can I be the cosignor on a credit card just for 6 months?
Not to my knowledge. However, while taking out a loan, you can clarify this with the concerned financial institution.
What I know is that later, after a certain time, you can remove your name but that may not be an easy affair.
Lender won't agree. I haven't met any such lender till date. Trust me. Why would a lender agree? I wouldn't have agreed in his case. In fact, I wouldn't have issued a loan if the cosignor doesn't have any confidence on the debtor.