I wish to lend insight into your question. I will try to stick mostly to largely incontrovertible facts in my attempt to do so. You have been on the forum for quite some time, and I am sure you have a great deal of personal experience with which to inform and evaluate what I offer below. I hope my reasoning and conclusions make sense.
Mastery abilities, which happen to be my favorite new addition in ICRPG Master Edition, are essentially “discovery leveling”; they are strictly a function of the aggregation of dice results keyed to a segment of a uniform distribution. They add an element of variability to the inevitability of character progression for characters who survive multiple sessions, which I personally find exciting and fun.
The award of each of three successive Mastery abilities is keyed to the occurrence of natural 20s on a D20 check, a 5% chance on each roll; when twenty 20s are collected, the ability is granted. This means on average, a typical player character will receive one Mastery ability after every 400 ability checks.
Therefore, the more ability checks that occur at your table during a typical gaming session, the faster your party is likely to gain Mastery abilities. For example, if a typical combat encounter at your table lasts about five rounds, and if you run about four encounters per session, you could expect every player participating in those combat encounters to roll one natural 20 approximately every session and to earn a Mastery ability by around the twentieth session. Obviously, some PCs will progress faster than this, and others will not.
Player engagement in your game will factor in to some degree. A player who takes more action during a session and makes more PC attempts per encounter makes more D20 rolls per session than a player who abides by taking idle turns, so you can reasonably expect the former to be ahead of the curve in collecting natural 20s and earning Mastery abilities. Therefore, if your players are the type to press their advantage through interacting with your game environment, they may progress faster in Mastery than players who absorb your game narrative more passively.
Furthermore, I will speculate plausibly from personal experience that at most tables combat encounters prompt more D20 rolls per encounter than exploratory (“puzzle”) encounters, which in turn prompt more D20 ability checks than heavy roleplay social encounters. If these assumptions hold, then the mix of encounter types that a GM runs over the course of a given campaign can and will affect the likely rate at which the players at that table earn Mastery abilities.
Therefore, much like a GM who uses challenge tuning (disruption, duration, and damage) to make adjustments to encounter difficulty, a GM can choose to adjust the mix of encounters to slow or hasten the rate at which players at the table are likely to earn Mastery abilities. A combat-heavy game with lots of rolls will tend to bring on Mastery abilities sooner, as will a game full of encounters that use a lot of disruptive environmental features, which prompt characters to make extra rolls in the form of DEX and CON saves. A GM looking to have PCs achieve Mastery at a more sedate pace can include more exploration encounters, where players state what how their PCs interact with the encounter environment but don’t necessarily make D20 ability checks to succeed, or the GM can include roleplay-focused encounters, where much of the game play is first-person dialogue to advance the story narrative with relatively few D20 rolls inserted only where necessary.
I am NOT suggesting this level of GM control; I merely state that it exists. Personally, I think a GM should adjust the mix of encounter types to serve the emerging narrative and create the most fun and excitement for the players, allowing the dice to hold sway over Mastery abilities, as it seems they were intended to do. In my opinion, I think it’s a smarter choice for us to consider and adjust our preconceptions regarding the “right” rate at which Mastery abilities are obtained. The, if new earned abilities create any issues in the game, the GM has many other tools available to adjust the sessions and the campaign accordingly to preserve the essence of the game.
My $0.02…
[ETA: After 36 sessions (usually about three hours each week), one player at my table is now several 20s into earning her third Mastery ability, and every PC has earned at least one. I notice that when a well-developed PC gets to the point of a earning a second Mastery ability, in combination with the abilities gained along a typical Milestone path during the same span of time, the power level of the character starts to get pretty impressive, which makes designing challenging a bit more demanding but also leads to some pretty epic scenes at the table.]