There isn't a "maximum duration" but if you have no experience, unless you want to try being a wholesaler, you have very little change of getting hired in.

You're talking two different horses here - I don't know of anyone who "underwrites" both general and life lines. Companies usually either sell one, or the other. When they DO sell both, the lines are strictly divided. I don't know of ANYONE who underwrites both.

Requirements vary by state. But getting the appropriate license is easy; getting a JOB in underwriting can be very, very hard. Getting free training from a company, means you have to have the job offer first.

Usually the job comes first, then the training, etc.

Unless you're a wholesaler, you don't NEED a license, btw.