I'm assuming that if I create a more experienced and powerful Gifted hedge wizard, as per Hedge Magic Revised Edition, I would use the rules for experienced Hermetic magi to determine how much experience they get per year for improving arts and abilities, learning spells etc?
have never tried it myself, but most Hedge Wizards don't get to do a lot of book studying.
So perhaps less experience.
@lvgreen is correct, but not for the reason he thinks. Hermetic Magi are unique, being supported by their Covenants, in getting 4 free seasons every year with no need to work.
Hedge wizards, in general, are built as Companions, and need to support themselves for two seasons every year. As such they get 15 experience points per year unless they are Wealthy or Poor. If they are supported by a Covenant, they cannot take Wealthy or Poor, but still need to work two seasons to support the Covenant.
Hedge wizards who are members of the Order, on the other hand (there are not many such anymore in 1220, but quite possibly some completely non-Hermetic ex Misc of various traditions) are supported by their Covenants, do get four free seasons per year, and therefore also 30 exp.
If your troupe agrees, the same could be possible for hedge wizards who are independent and well-supported enough that they don't need to make any effort to support themselves - not just Wealthy but totally without time-consuming obligations. This would be a "house rule", there's no defined way to accomplish this mechanically, but there is no logical reason why it should be possible for magi but not for others who are similarly outside of medieval society, other than the meta reason that Hermetic magi are the focal characters of the game line.
Gifted companions are created as companions (HoH:S p. 107). This is made explicit e.g. for Folk Witches in HM p. 34:
"To represent the folk witch apprenticeship, you may spend 200 experience points for the character on any Abilities. This is less than a magus receives because a folk witch apprenticeship is shorter and folk witches do not have such a systematic and organized method of training. Following her apprenticeship, a folk witch is assigned the normal number of experience points for a character each year; that is, 15 XP each year, although this may be affected by the Wealthy Virtue or Poor Flaw."
For the 10-year Gruagach apprenticeship (HM p. 56):
"The gruagach apprenticeship grants the character 160 experience points for any Abilities, including Arcane ones or the gruagach Arts, and 80 levels of spells. After the gruagach apprenticeship, the character accumulates experience using the standard rules."
For Learned magicians (HM p.79f):
"Learned magicians receive experience points using the normal character generation rules before apprenticeship." and "Each year that the learned magician trains under a master he gains 30 experience points."
An exception are the Vitkir, who are designed like magi (p. 120).
Side note: Don't confuse a hedge wizard for a magus Ex Miscellanea. These two groups are similar, and easily confused.
Hedge wizard: Magic professional, not a member of the Order, using a traditional magic method.
Magus Ex Miscellanea: Magic professional, member of the Order, using a traditional magic, Hermetic magic, or some hybrid.
Mmm, true. When I say "hedge wizard" above, my focus is on the "traditional magic method" - someone who has magic who has not been Opened to the Hermetic Arts. I ask any Gruagachan or Sahirs who might happen to have taken the Oath and learned the Parma, to please bear with me. 
I'm thinking about viktir actually. In the book it doesn't appear to specify how many xp they should get per season if they're more experienced, and I'm not all that sure how they go about learning some of those rune scripts in basic character creation either, as a lot of them tend to be quite powerful and hard to learn without high rune scores. It looks like they only get to add one rune score to the total for learning them and no +3 bonus that hermetic magi get.
HM says on p. 120 "Vitkir characters are designed like magi, with only a few exceptions." So to me it's clear they get 30xp an 2 warping points per year after apprenticeship, just like magi. They will of course take the mage slot (not the companion slot), so you can't play both a Hermetic mage and a Vitkir.
There is a Major/Minor Rune Focus Virtue and you can take Puissant Ability (+2) in a Rune. Yes, they only add one Rune, but Runes are accelerated abilities, so they progress like Arts. They will learn 120 points of rune scripts during apprenticeship, as described in the section "Vitkir as characters".