DJ# 8 covers this a bit. While some of the thinking on things can change, just like any still-in-development project, it'll give you an idea at least. Sorry for the wall of text. Thought it would be useful to put here. I'll include a link at the end if you want to read the whole DJ.
Skills Mastery in Chronicles of Elyria is divided up into tiers with a percent mastery for each tier. The tiers are broken down as follows:
Novice (0% - 29%)
Apprentice (30% - 39%)
Journeyman (40% - 49%)
Expert (50% - 59%)
Artisan (60% - 69%)
Renowned (70% - 79%)
Master (80% - 89%)
Grandmaster (90% - 99%)
Legendary (100%)
The percentage mastery reflects how proficient you are with a specific skill but also inversely reflects the percentage of people who will attain a specific skill tier. In other words, based on the amount of time it takes to level a skill, you can expect distribution of mastery across a server to look like the following:
Novice: 100% of all people can achieve Novice in any given skill
Apprentice: 70% of all people are likely to achieve apprentice in a skill
Journeyman: 60% of all people will achieve journeyman
Expert: 50% of all people will achieve Expert
Artisan: 40% of all people will be Artisans
Renowned: 30% of all people will achieve Renowned
Master: 20% of all people will achieve Master
Grandmaster: 10% of all people will achieve Grandmaster
Legendary : Approx. 1% will achieve Legendary
Put differently, we calculated how long it would take people to reach Legendary status over several lifetimes if they did nothing but work on advancing a skill. Obviously, that's not going to happen. So as people train and improve, the Soulborn Engine monitors progress and provides bonuses as necessary to ensure the top 1%, 10%, etc... will eventually reach the Legendary or Grandmaster tier.
You should also note those skill tiers carry across reincarnations and it will, in general, take a lifetime to become a Master, multiple to become a Grandmaster, and several to become Legendary.
DJ #8: Character Roles and Skill Advancement