I've currently used a week in my house rules for the initial learning of an advanced skill - this may be a bit short though for some of the skills which inherently would seem to need extensive grounding.
I might be a bit harsher and go for 2 weeks or maybe even 4.
What prompted the question were the new Sorcery rules where each spell and each sorcery skill are treated as advanced skills. I'm looking for a value where its not too easy to learn sorcery but trying not to penalise sorcerers too much
Following on from that the existing rules state that advanced skills can be taught by a mentor but there's no indication as to how skillful that mentor should be or whether the mentor should make a skill roll (as in experience rolls). I'm thinking of requiring the mentor to have at leats 50% in the relevant skill but not require them to roll.
Mhm. Forgot that the starting skill for an advance skill is higher than the old 1D6+modefier. 2 weeks might be more appropriate taking that into consideration.
My old houserules were that a trainer needed above 75% to teach, and that you couldn't train a skill higher than 75%, the rest had to be through experience (except for knowledge skills, which could be trained to 100%). That way you avoided rich adventurers taking years off to train their skill skyhigh.