Mastery happens, first and foremost, when I decide it does. That said, my internal process for deciding it is something like this:
a) when a crit happens;
b) when it is, in my opinion, narratively appropriate for a spell to be mastered; and
c) when there's no more pressing issue that the benefits of a crit would be better applied to.
If anyone is ever even a little bit tempted to point to a past or future situation where a crit did not result in a mastery, and argue that it should have, I would encourage them in the strongest possible terms to instead not. I try my best to be an easy-going kind of person, but very little gets me more heated than someone trying to 'gotcha' me. The relationship I want to have with you, the readers, is a collaborative one where we all work together to forge a story. If it turns into an adversarial one where readers feel like they can argue me into giving Mathilde extra benefits, all it will result in is undermining my desire to spend time here.