Thanks for your answers, there's a number of good suggestions.
So far, my mage follows the oxfordian doctrine, meaning it's a pain to cast a spell during a fight, but if I have time enough, I can reach TN 100+ with the increase AP immuto, stacking buffs and making them effective for a year is easy, provided I have let's say a couple of hours of free time, once a year.
Oxfordian doctrine was an easy and logic peak, as I play a Mage, and the most renowed ones are the oxfordian mages. Furthermore, it gives me that suit I desperately need to cast my spells, as my carrer doesn't provide any.
According to the v2 rules, it's ok to be an oxfordian mage even if the guild doesn't like you much, and I don't see what's wrong with enhancing or protecting yourself, malifaux is a dangerous place, and it's not like I'm going to throw fireballs around.
As for the counterspell, sure it will reduce the duration of my buffs, but going from 1 year to 1 month, isn't going to hurt much.
The major issue isn't the duration, it's the fact that the spells are inherentely too powerful and modifying the TN isn't going to change anything for an oxfordian mage.
I like the idea of maximum +2 instances of duration, even if it would hurt my character a lot.
The same goes for the +3TN / + to all spells casts while there is still an instance of the spell active, that's a good idea.