I can't remember but i think that a book (core rulebook or covenant?) states that a normal magus is expected to give 1 of his 4 yearly seasons in service times for the covenant (adventures, vis source recolting, items making, etc) and, thus, to have 3 free seasons by year.
I don't recall anything in RAW about this. The sample covenant oath in Covenants (available at http://www.atlas-games.com/pdf_storage/ArM5CovenantOath.pdf), under Responsibilities, hints at the variety of ways in which individual covenants in your saga might address covenant service.
The only half-way official rule on this I can remember is the charta of Triamore, which demands at least one service in five years from every magus. But in the Rhine Tribunal there is this regulation for Peregrinatores (wanderers) who can stay at a covenant as guests for some years. For them the minimum fee is 3 pawns of vis or a season per year (if I remember correctly - serf's Parma).
I don't think there are any rules per se for this, but as mentioned it is a common occurence in Covenant charters to ask for 1 season of service for the common good per year.
In my sagas we've always done the 1 season per year thing in spring covenants, and sometimes stopped service entirely or changed to once per 2 years.
for my recent sagas, I have been suggesting 2 seasons per two years. so that if a mage comes up with a two season project for the covenant, they are covered or if they are in middle of a project that takes four or five seasons, they aren't interrupted and can complete the project before taking up duties.
The mage that supervises the mundanes of the covenant during a year (usually an hour or so a day and uses up all the free days of every season (and sometimes a few more)) get credit for having served their season for the year. (My maga usually ends up doing the job since as Mentem specialist, she gets to the root of issues very rapidly among the covenant grogs/villagers and can deal with it, esp since 1/3 those that live in the covenant itself are her relatives.)
Our covenant requires one season of fairly specific service from each magus between the end of tribunal and the start of the next tribunal, and a magus can 'buy off' the season by paying a 10p fine. Anything requiring service from the magi outside of that is done by the council bribing the member magi with vis.
That said, the 'bribes' are the only way the magi can actually legitimately get vis from covenant stores - basically if you want from the covenant for your own personal projects you have to earn it.