As to how long they take, this is a question that has become impossible to answer. We used to give an average time, but that average time was one month, then two months, and now we have seen them take as long as seven months.
This is frustrating for sellers and can be a real business killer, but there is no way to expedite them. This is Amazon's system and getting your account back is not something you can get by suing them in arbitration. Washington courts have ruled Amazon's business solutions agreement is an agreement at will. They can cancel it at any time and for any reason.
I know this is not what sellers want to hear, but worrying about it will not speed up the process. The most important thing to remember is to have the best plan of action possible, because this is your last shot. For that, we can certainly help.