If Atkins constituted a stall to be 6 weeks without scale weight loss or measurement decreases, then I would think it would be the same or similar for maintenance.
When you've actually maintained for 6 weeks with no losses or gains, then I'd say you're 'there'... or at least that's how it was for me.
At the 6 week mark of maintaining the same weight, I guess you could then retroactively begin the 'count' from when you last lost/gained the 6 weeks prior, and count from there.
Have you also been measuring?