So, my mind right now straddles the line somewhere between staff and player. I can see the ins and outs of this from both sides pretty clearly. Gathering from what I believe are the most important takeaways posted here so far, the following is what I would propose:
Learning Times:
1x: On Demand
2x: Two Months
3x: Three Months
4x: Nine Months
5x: Fourteen Months
Monthly Cap:
Any three Ability purchases, at any level.
Teachers:
One may teach an Ability at any level less than or equal to their own, regardless of Ability.
Without a teacher, one may learn any Ability up to x3, with justification, subject to staff approval.
Learning an Ability at x4 or x5 always requires a teacher, or double the learning time, with significant alternate sources of instruction, subject to staff pre-approval.
Certain rare or specialized Abilities, at staff discretion, may always require a teacher regardless of circumstances.
As a note, teaching and learning through roleplay should always be encouraged. I imagine it ought to be one of the factors that earns you (or lacking, doesn't earn you) Good Roleplaying XP each month. To be clear, I am not advocating that Good Roleplaying XP awards be judged solely on teacher vs. teacher-less learning or instruction.