There's no set answer. If you want your characters to use lots of vis, you need to give them lots. If you want them to only use a little bit then only give them a little bit.
It is probably best to start out being conservative. Only give out a small quantity and see whether you and your players feel unduly constrained by the amount of vis the characters have. It is a resource for the characters, so you probably want to have the characters having slightly less vis than they would really want (so, that they need to make decisions about how to use it). But not so little that the players are frustrated by seemingly able to achieve nothing.
Remember that a character can extract vim vis from the aura equal to his Creo Vim Lab Total divided by 10. So, this can be used as a kind of standard. Say, a character can extract 3 pawns of vim vis from the aura. Then a story in which he gains 1 pawn of vis is a bit of a waste of time, for the character from a vis perspective. But of course, he may have other reasons for being interested in the story (gaining a book, or allies, or not dying, or achieving other goals, or whatever). On the other hand, a story in which he gains 12 pawns of vis is a real vis bonanza; it's a whole year worth of vis extraction.
As others have said a character can only take advantage of one experience source in a season. So, if he earns 10 XP in a two-day adventure, and also reads a book for 10 XP in that season, then the player has to choose which source the character collects the XP from, either the 10 adventure XP or the 10 book XP. He can't benefit from both.
It's better, I think, to give out the vis that seems appropriate to whatever the characters are doing. If they are busy slaying sea-monsters, for example, then they are probably going to acquire Aquam vis from the monster corpses. If this isn't what the characters need, then the player characters can try to trade with magi at other covenants. Alternatively, if, say the characters really want some Perdo vis, then you can allow the characters to make (say) Magic Lore rolls to figure out what types of beasts or what types of locations might contain Perdo vis. So, then the player characters can go out to try to hunt the creatures or visit the locations appropriate to the sorts of vis they need.
Also, don't forget about Confidence Points. Characters get Confidence Points from adventures too --- usually for successfully overcoming challenges and so forth.
For what it is worth, in our saga we run at an average rate of one story per magus per year of in-game time. But it does change a bit from year to year, depending on what is happening. Sometimes a character has a big project and spends the whole year sitting in his lab. Other times a magus interferes in a story "intended" for another magus (or is called upon to help), and so ends up involved in several stories in a year.