Well, there is this...
...which is the first sentence of the apprenticeship section of the character design chapter.
It doesn't say that that's carved in stone in the list of the Ars Magica commandments, but I think it's clear enough to cover pretty much all cases (also on that chapter there isn't also any reference to the childhood's 5 years to be mandatory). Of course then there can be issues, like Tytali slaying their parents and thus being considered gauntled and welcomed by the rest of the house, or magi dying while being parens, or Tormenting Parens that think character's apprenticeship isn't over, or whatever.
IMS if anyone comes with a character design claiming that their apprenticheship lasted anything different to 15 years I would just require the 15 years XPs and spell levels to be spent as in the character creation rules, and then just add whatever they say to justify that as character background story. You are a Tytalus that killed his master at age of 13, after 3 years of training? Cool! Then show me you picked the Poor Parens flaw, that you have a cool reputation in the house and mechanically just consider that your apprenticeship lasted 15 years anyway, with you spending most of the time learning stuff on your own, replacing seasons working for your master with seasons spent servicing other magi to gain access to their teachings or their library.
Besides that I think the 15 years rule will probably be in each Tribunal's Pheriferical Codes. It balances the service magi can expect from apprentiches, ensures them 15 seasons of study and more important gives the order 15 years to decide if that young punk deserves to be taught Parma or to be burnt down on the spot.