or until they run out of money
As long as they don't rely on foreign supply for their missiles[1], they can manufacture them forever without having to pay anything to anybody.
[1]: it may sound like a bold hypothesis, but I doubt anyone would ship missile materials to North Korea. China could, of course, but that would be a bit risky diplomatically.
Historically, Iran, North Korea, and Pakistan cooperated on nuclear and missile tech, though I think that was engineering rather than parts.
And with NK slaves showing up in Polish shipyards, that is highly unlikely.