Dan just commented elsewhere on Fairmath in a way that sheds light on this question:
So if the player is given enough opportunities to affect a given stat by %+/-10, then one advantage of Fairmath is not needing to keep track. You can trust that a player who makes consistent stat choices will cross 75 at some point, a reasonably consistent player around 60-65, a “leaning” player 55, and vice versa on the other side of the scale.
%+/-10 is pretty reasonable for a standard bump. Maybe %+5 if you have a lot of choices and want every single one of them to matter (because it pretty quickly turns into +1/+2 to a strong stat, which is a pretty slow accrual rate).
Of course, there are cases where higher changes are appropriate. IIRC there’s a point in my game where you can get %+50% to ruthlessness, which should be enough to cement an already-ruthless reputation while completely gutting a reputation for being compassionate.