Well I have a different perspective on this, since I train once a week with a Kru and twice a week at my university with a group of students from all sorts of martial arts, mostly kickboxing and muay thai, but also some students of tae kwon do. We run a very loose society, where we encourage free sparring, exchanging techniques and tips, and although no one is in charge or conducting the class, we get a lot done and we all feel as if we've learned a lot from the experience.
Of course, quite a few of us have more than 5 years of MA experience under our belts, so we are also aware of maintaining proper safety, which I think is the crucial thing for any martial arts instruction. So I guess the amount of years training isn't as important as your style of training, and the only thing that is absolutely essential is proper safety training.
This may be a tough one to answer. I think there can many factors that come into play. It depends on your skill, knowledge, your ability to train others and your drive. Some schools require fights to advance to instructor levels and some of the best instructors never fought. Personally I don't think the amount of years training is much of a factor. When I became an instructor at my old gym, I hadn't been there long at all and I advanced over people that had been forever. But I trained daily, for a minimum of 4 hours. I listened and paid attention, worked on every technique to get it right and just the hell out of myself. In the end it paid off. I think you and your instructor (kru) will both know when its time to become a instructor. As for myself, I have been involved in muay thai for a long time and I am a certified instructor (recognized by WMC, IMTA)under the founder of my old gym but there is still more to go. Remember this, no matter what level you attain, never stop learning and always enjoy it.