Well actually its not like a linear increase in knowlege and years playing= higher level of guitar playing.
Level 9/10- You have to create god like unique music with the guitar. You could be level 10 after 1 year or 50 years. You could know 1 scale or 50 scales....ITs irrelevant.
It's also very subjective, so the ability of a guitarist is somewhat irrelevant completely after a certain level because it doesn't matter how good they are in terms of technical proficiency when some one doesn't like their music.