Consider a 0th level character, or a character with existing character levels, multi-classing into a new one.
I think, she needs to go somewhere – to a fighters’ school, or to a wizards guild, etc – and need to pass a training. It is highly world-, class- and DM-dependent.
But how long time does it take?
I ask this in the hope, that there is a rule, or at least a hint in a rulebook. What needs to be done to get a new class, it varies, but how long does it take, I think it has (or should have) at least a hint somewhere. I don’t know even the magnitude – is it some weeks, or decades?