How powerful must one be to qualify for these titles?
Guardians of the forest say hermetic reputation of 2 for magister and 4 for archimagus
I want an idea of skill in arts
I think that either gardiens of the forrest or HoH true lineages has the requirements for becomming an archmage.
For the other titles the book gives a general idea regarding magus age past aprrenticeship doesn't it. I'd recommed you comapare that age to the standard magus advancement from the character creation system and sort of back calculate what the numbers would look like. (and then post your results here so that we're all spared the work 
)
On page 24 of Guardians of the Forest it has the rules to become archwizard. I'll recap.
You need to have created a spell of 7th magnitude (35th level) or greater, you must have performed a feat that is known throughout the Order, and if you live in the Rhine you must be a Master. In previous rules editions youalso had to have trained an Apprentice. From there you have to challenge an existing an Archmagus and win. Each Archwizard has their own challenge, which stays the same from the first time they issue it.
As far as what skills are needed, difficult to say. The 35th level spell isn't all that difficult to create, the feat that the whole Order recognizes is a bit tougher. Challenging an Archmaster (it is customary to challenge your own Parens first if he is an Archmaster) is the tough part. Their challenges might be as simple as beating them in Certamen (Many Tremere Archmasters have this) to finding exotic magic items and relics, to an actual fight with them or a poweful beast. In our campaign one of the newest Archwizards was given the challenge that he had to "Grapple with a dragon" which meant he could not Target the dragon with any magic, all spells had to be Range Personal.
I'd say that anyone who thinks they should be an archwizard should probably be able to pull off a spell casting total of 50+ before any dice are rolled at least. and that's the low end. A few Arts should be 30 or more.
1 Like
Guardians gives a list of feats to qualify...one of which is invest a device or invent a spell of at least 5th magnitude for magister. 7th magnitude for archmagus.
Since you can do this over multiple seasons you need a lab total over 25...which may be hard to obtain if you generalize over 20 yrs (see my next comment)
"petition for masterhood is helped by a number of other factors....include having a familiar, a reputation for politics or famus deeds, having won a wizards war...is rare to reach this level in less than 20yrs"
20yrs based on character creation is very different from 20yrs of me playing my character since I tend to study more and invent less...helps when you dont have to know the spells 
based on 30exp a year for 20yrs you get ~8-10 in all arts...not very impressive... if you specialize then you'll just have the skills to train an apprentice (5 in all arts)
Some of these elements are likely left open so that one can accomodate the nature of your particular campaign. In a high fantasy game perhaps 10 years of study is all you need, in a more realist setting perhaps 50+ years.
At the old forum and perhaps this one we discussed Arch Magi and cronysism. The process to become an Arch Magus can move beyond reaching a particular statistical level.
What I would consider is what this status means to the character, the troupe, and to your own going stories. If this is a life long dream and the campaign is coming to an end, petition the troupe to allow you to achieve your characters dream.
If you imagine playing for a year or more, what changes will occur if one of the magi in the troupe is an Arch Magus? Will he have more clout and say than the other players? Will resentments occur between the players? Or will this open a new avenue of storypaths for your character as well as the troupe? I think the troupe should be discussed as one characters transcendance to the status of Arch Magus can have campaign shaking reprocussions.
Good luck.