Can I be the cosignor on a credit card just for 6 months?

No there is no such rule. However, the lending rules varied from lender to lender. So, before signing a loan as a co-signer, ask all the details.

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.

The possibility is nil my friend. The lender won't agree on such terms where the co-signer doesn't have faith upon the borrower.