There is no official MPGH duration for warranty. Seller decides his warranty on his own in his thread. If warranty is stated something like this "1 week warranty for any limitations/problems with the account. After that i don't hold any responsibility." then if it stops working after 2 weeks seller is not at fault. You can also offer no warranty what so ever.(Only guarantee that account will work on log in) but you have to be explicitly clear about it.
That being said, you can't just write anything in your warranty/tos information. It is reviewed by marketplace staff when approving(For example lifetime warranty is not allowed). And for example if you say 1 week warranty but you do something to make the account stop working after 1 week it's still your fault.
Terms of Service on threads and rules can be overridden by MPGH Staff decision in scam reports in order to prevent unfair treatment, and/or if absolutely necessary. @Sadorusk