We're now at the stage of our Rhine Gorge saga where the magi are about to put down roots at their chosen site and I'm trying to develop the possible vis sources nearby. However, as this is my first saga, I'm not really sure what would be a reasonable amount of vis to allow them. If I were to go straight down-the-middle (ie: neither a Low Vis nor a High Vis saga) what might be a reasonable number of pawns of vis per year for three magi who have just passed their Gauntlet? I do understand that this is a very "how long is a piece of string" question but I don't currently have any experience to draw upon to guide my hand.
First, work out how many pawns per year they will want to spend on their Aegis of the Hearth (minimum 4).
Second, do they have anything that will increase consumption - Verditius are likely to want more Vim vis for opening greater items for enchantment, someone may have Waster of Vis as a flaw, someone may have Free Study as a virtue and be keen to use it, people with "age quickly" or "difficult longevity ritual" may throw more into their longevity rituals.
Third, decide on how you're going to adjust it when the saga requires more of less vis. I find start low and increase if they run short tends to be easier than giving out big vis sources and having them destroyed/raided if you think they're getting too much. People complaining "we never get enough vis" and then getting an adventure opportunity to win another source looks like the saga is responding to player demands; giving a big source and then having a church get built on top of their favourite creates an impression of "guard everything and prepare to have it all taken away".
Start with 3-4 pawns per year per mage on top of the Aegis requirements and see how you go from there.
The Greater Alps Tribunal requires that any covenant have 10 pawns a year for every magus at the covenant, and this is established as "wealthy." Since your goal is average wealth, the covenant would have less than this. 5 pawns a year is "poor", so you're aiming somewhere in between, perhaps 7-8 pawns per magus: 21-24 pawns per year for your covenant of three.
Even this small amount of vis will begin to accumulate, however. Newly gauntleted Magi typically have modest Magic Theory and can't spend large amounts in a single season. They're busy with all the demands of establishing a new covenant, and they also want to read books to improve their Arts. They want Talismans, but they're probably a decade out from needing Longevity Rituals. You might put some thought into offering ways for the Magi to spend the vis they will slowly accumulate: consider how much vis you want to charge for books, offer some lesser enchanted items for vis, and don't forget that vis can be exchanged via House Mercere, but the rate is 2:1. All of this will keep the Magi hungry for vis and looking for more, which is what you want at this early stage.
All very useful, thank you. I had, of course, entirely forgotten about the Aegis, for a start!
I don't believe that any of the three magi are likely to be heavy-duty users of vis as regards lab work. One of them is a healer, however, so the availability of Creo or Corpus vis will certainly be important to him (whether I supply any is another matter!). I do want the scarcity of vis to be a noticeable factor in the game so it will certainly be better to start with a low amount and increase it later as needed.
I had assumed that they wouldn't be needing a huge amount to begin with but they do need to do a tour of all of the Rhine covenants and get a backer from each one. So some vis for bribes might well be potentially useful! 
If you've got a healer who is going to want a dedicated source of Corpus vis, take the opportunity to come up with a really interesting source and make it the center of an early adventure. Then you'll have to protect it against raiding fairies, nurture it in the hope of making it grow, and perhaps even fend off legal claims when a magus from an old covenant in the Gorge tries to say he still has rights to it.
I second darkwing
At the founding of a covenant, they must not have any vis-source at all. Finding and harvesting the covenants vis-sources is a great way for new players and SGs to start a saga. It gives you some little adventures right from the start and for the players it is nice to find out what their Magi are capable of and to get used to magic system (especially) spontanous magic (since every vis-source would be discovered and harvested differently). Nice examples of vis-sources you can find in Covenants book (p.72 ff)
Also very important in that stage are the grogs and how they collect those precious pawns of vis.. since later on magi, normally don't deal with such "dirty work". 
Finding ways to make grogs capable of independently harvest the covenants vis sources without disturbing the magi all the time, is IMHO in fact one of the first goals of a spring covenant. at least in my Saga.
regarding the amount of vis: the less vis the magi have, the more they are happy when they find some. Therfore, it's IMO better to start with a small number of small vis-sources, but instead let them find more of raw vis (as rewards), while they are searching the "sources" behind it. This process could also take some seasons or years along the way as you play the first story arcs.
Have fun with your first sessions!
It's a quite useful thing for starting covenant stories.
In our starting saga the players are setting a covenant in the Isle of Man, were any previous attempt had tragically failed (except for some Diedne coventants that back in the time prospered there and then were burnt down during the Schism War). After the last one even a warning was sent to the Order commanding not to try that again. After some scouting and research they found that the problem is one of the most powerful dragons of the island, who deeply hates hermetic magi and kills them on sight. So they had to have a quite powerful reason to move to such a dangerous place and plan some preventive defenses. The reason to go was that they knew of a spot with a yummy magical Aura of +8, quite interesting for them as they use to cast spells from afar through Intangible Tunnels. And the defense plan was to buy with the covenant build points a casting tablet with an Aegis ritual of 50. Their grasped their hands thinking 'problem solved!', but that's were the fun begun: the ritual requires them to burn 10 pawns of vim vis per year, and they have only one vim source, so they have to spend one season per year and magus gathering vis from the aura only to gather the vis required to cast the ritual. And now sooner or later the dragon will find about them and, unable to burn the covenant down, lay siege on it, and then they won't be able to go out to gather the required vis (or get any contact with Mercere magi to get stuff, news and so), and things will get quite interesting.
Are you using the Isle of Man resources from Hermes Portal?
Yes, of course we are, though there are some things there that I find akward, like for example why didn't Mannannan repel the norse invaders, or how the hell did the fae manage to conquest an island before humans arrived. It seems to be somehow against the fae paradigm of the 5th edition (of fae being pretty much human vitality parasites), but I think it's ok, as I never liked that perspective of fae doing nothing at all when there aren't any humans to play with around.
In fact that non-human-related activity could be some pretty nice to investigate by our Merinita.