Assume the following:
Flaws such as difficult/weak spontaneous, deficient tech/form, weak enchanter, and similar flaws are a result of incomplete training durring apprentice ship.
If you could find a master to show you then you could get enough new training to "fix" the flaw.
How much training do you think it would take. obviously the major and minor would take different amount. And you'd probabaly step up the levels, so a major woud change to a minor, and then with more training finally be fixed.
As much training as is required to earn the Minor Virtue or Major Virtue that your Flaw paid for, as defined by the appropriate set of rules for gaining virtues, like the Mystery system, or the Labor system in City and Gild.
The big part of learning a virtue involves a mystry cult lore + pre. That doesn't apply in this case, so you'd have to take 15 points of ordeals that are essentially time sacrifice since all your being is retrained....that doesn't seem reasonable
Getting free Virtues doesn't seem reasonable to me either...which is the end result of allowing a character to easily train away Flaws. If one of my players wanted to discard a Flaw, I'd allow it only at the expense of gaining a new, different Flaw or losing a Virtue of equal worth. There is no such thing as a free lunch.
a) Not every flaw should be able to be fixed, some should be permanent, but I don't think that explicit rules about which ones are permanent and which are fixable are needed. Exactly which ones can be fixed should vary from character to character --- for one character a deficient technique might be fixable, for another it might not be. As long as the troupe can agree it should be fine, but bear in mind that it might not be obvious to the characters whether or not a particular flaw can be fixed. So they might waste their time trying to fix what cannot be fixed.
b) It shouldn't be easy, or routine to fix the flaws. At the very least it should be time consuming, otherwise no character would have these flaws.
c) These are supernatural/hermetic flaws, so there should be supernatural solutions to solving them. The solution shouldn't be as mundane as doing more study, or finding another teacher.
d) The troupe should see a character trying to fix a flaw as an opportunity to tell a story. It's not about a player trying to get a "free virtue" or whatever. Characters trying to overcome their flaws is a very sensible and natural way to develop your saga: it's much better than hanging out at the inn waiting for a mysterious stranger to arrive. A clever story-guide could develop lots of interesting stories about the things that the character needs to find, the deals he needs to make, the places he needs to visit in order to overcome his flaws.
e) Largely because of the story-telling opportunity my suggestion would be some sort of quest, with a bit of laboratory work initially required to discover what the quest is. You could use the Mystery Cult ideas as a model, or indeed the character could find and join a Mystery Cult that solved the problem. Another (more lab, less story) idea could be to use the Original Research rules, with it being a relatively minor research project to fix a character's flaw (as it's not an error in Hermetic theory, but the character's own practise that is being corrected). But as long as you can tell an interesting story I wouldn't get too hung up on rules for it --- or whether it "balances" or whatever.
f) I think it should take at least one (in-game) year of activities to remove a minor flaw and a bit of risk and uncertainty. Obviously, longer and/or riskier for a major flaw. The activities might include laboratory work, adventures, etc. But this is just for the speed of saga that we run...one in-game year, would take us about one-two months of real-time...a different sort of time frame might be appropriate for your troupe.
It's identical to earning the removal of your flaw as part of a mystery, though. Amd I'm not saying you actually need ot get the Cult Lore, just that you need to put in character effort of equivalent difficulty.