I think that the house virtues should be part of the apprentice training (in one way or another) and that you should get the benefit when you pass your gauntlett.
I also really like the idea of granting virtues when somebody reaches a very high ability score. The example of the librarian who got book learner when he went to 7th (or so) level in profession librarian is wonderfull.
There are virtues (and flaws more so) to be gained in adventures. (take crippled or guardian angel) Those should not cost time.
But if somebody really wants to learn a specific virtue, I suggest basing the costs on mystery virtues. If one of the magi wants to learn Harnessed Magic for instance, he should learn the ins and outs from another, spend some seasons and practise a lot. (to keep game balance, but still allow advancement in other ways than just increasing arts)
A virtue like Fairy Blood is probably inattainable after character creation.
As a side question (which is mostly relevent, because it discusses the relative oomph of virtues), would you charge more time for Virtues which grant bonuses (boni?) or additional options to labwork or to spell casting?
Things like Hermetic Theurgy or Hermetic Alchemy (which, for my two cents, should be easily taught as they fit within the hermetic framework nicely and break no limits) don't actually grant any power to a magus. They just open options. Puissant Ignem, on the other hand, is an immediate boost in power across a broad range of things.
Probably not, but that may change. This option is still used on a "trial basis" sofar.
At the same time, you can easily argue that additional options also results in an immediate power boost.
But i certainly see your point. Sofar i think it better to keep it stratified though rather than different "price" for different Virtues.
Otherwise you will also have to face the question of indirect values, like if its possible to learn one of the study bonus virtues, should it then cost more because its longterm effect means a much more powerful magi, or not exactly because it is a longterm effect and still requires study to be effective?