Between 16 and 20 weeks is ideal for me. If you can handle the crowing and the general making a nuisance of themselves, 6 months isn't too bad either. More meat and I actually like it a bit tougher, which isn't really that pronounced if you cook it right. I used to process my brahma's at 9 months because by then they'd have a decent amount of meat on their large frames. At 6 months they were still skin over bone. For a good coq au vin, a rooster of at least a couple of years old is ideal in my opinion, but taste is very subjective. See what you like, and grow and process according to that.
I think for a lot of people it's a cost factor also. Which is completely understandable and logic. If you can have them forage some of their own food, they will grow slower and it may or may not cut the feed cost because you'll be feeding them longer. If time is no constraint that's an option, but you will of course be processing later and the meat will be tougher and 'tastier'. They also tend to end up not as bulky as when you only feed them growing mix.
I read a story somewhere about icelandic chickens being completely self-sustaining, which is great if you have the space to do that. You will still end up with a small chicken though. But who cares if meat and eggs are relatively free, right? Land has its price and you will be processing several birds instead of one CX, there's a cost factor in that too. So, that depends on your circumstances and preferences.
I feed my birds lots of greens that I grow in the garden and table scraps of course, on a daily basis. I often give them access to a part of the lawn that's fenced off, since I'm limited in space it's figuring out a balance in letting them forage and letting the patch recover. They get very active on that patch, especially when they're still young, so they probably burn more calories than they find there. They also get in destructor-mode on that patch, hence trying to balance the destruction and recovery. But I want to eat a healthy and tasty chicken that is a bit tougher and redder than the broiler processed at 8 to 12 weeks. For me that's worth the extra time and feeding cost for a skinnier bird compared to a fast growing broiler.