Depends on a lot of factors.
I've seen guys that have trained for years that are still so inept that the training hinders them with a false sense of security and reality. They're still so afraid of hitting the heavy bag that they self-identify as prey in any sort of combat later on in the gym. Good for them for having a passion and staying fit. But the answer for them is "never".
I've also seen natural born killers who just randomly show up and start doing two-a-days 5 or 6 days a week. Super fit, super aggressive, super fast learners. One month.
But both types are rare.
I'm not sure the once-or-twice per week crowd ever really get there. Anyone who goes after it, though... they're good enough to get themselves into trouble at 3 months, good enough to get themselves out of trouble at 6 months.