I am not there yet but close!
You'll know when it's time.
In-between the anxiety of it looking bad, at the same time there is an incredible sense of freedom that makes it worth it, if that makes sense. Freedom from the salon, more free time to do other things, more money to spend on other things, freedom from the shackles of societal expectations. Whenever I start to doubt my decision, I remind myself of how much of a slave I was to that 6-8 week touch-up schedule, and how much time and money I spent on that when I could have been doing other things. It was like an addiction - I'd feel great because my hair would look great right after a salon visit, but that only lasted a few weeks, and then I'd be compelled to go in for the touch-up. I actually started to resent my stylist (my "dealer") toward the end because I felt so abused and exploited by that whole cycle of addiction. It's weird, but that's kind of how I see that whole situation now.