Hey man, I'm in a very similar situation than you.
I've been playing for almost 10 years now. The first two, I took lessons. Then I "learned on my own" (that's what I thought, looking back, I never practised anything valuable properly). At some point, maybe year 5 or 6, I got a double pedal, so I went three months to a teacher to give me some exercises for that, and then continued "to learn on my own".
I'm not a professional player, by any means, but I've played in many bands, I've recorded and gigged. I've been a sub for many drummers too, and I get reccomended and called by many musos. So yeah, I think I can handle myself pretty good (when it comes to playing in bands, at least).
Anyway, I think that I hit a wall the day I realised that guys that had been playing for less than I did, could play better than me. What was the main difference? All of those guys studied for all that time, had instant feedback from a pro and worked on their weaknesses, whilst I played mostly the same stuff all the time, learning new things just by trial and error.
Nowadays I'm taking lessons again, for almost a year now. And man, it's so great. I'm working on all that stuff that I never worked on and I feel like I made more progress in the last 6 months than I did in 4 years of playing by myself. Following my own teacher's advise, I only go twice a month, once every two weeks. That allows him to give me more stuff to practice and me more time to do so. Maybe you could do the same.
Just make sure you find someone that you admire his playing, who knows how to teach and also that you'd like to hang out as a friend too.
Edit: to answer the question... I think you should stop taking lessons when you feel you don't need them anymore (when you feel there's nothing you can't play), or when don't have fun or you feel their cost is bigger than their benefit, I guess. But make sure you think twice, you may regret quitting later on.
Cheers!