The magi found a plant that had a few pawns of vis in it. The were going to extract the vis and then paused and wondered if extracting all the vis would kill the plant (it would have). Then they wondered, since the plant is a living thing, maybe the vis will replenish if they don't take all of it (it will). So we set about trying to figure out how the madi could determine the answer to these questions.
There was debate as to whether this could be determined by a spell or would need to be done as a season's activity as per studying an enchanted item. While we all agreed the spell would be InVi, there was some debate as to whether a single spell would suffice, or if there needed to be a version of the spell for each art.
Well, just keeping the plant under observation (a quick peek every day, not a full lab activity) for a few month after taking part of the vis should give a good idea of whether the vis is replenish, and how fast. No spell, no seasonal activity, but it would take a season (or more, if it replenishes very slowly).
If they want the answer now, there the InHe lvl 1 guideline "know whether a given action would harm a plant" that seems appropriate as far as knowing whether taking all the vis would kill the plant, and whether taking part of it would (two separate casting of the spell). It is still a plant, so that should be enough, but if you want to make it more difficult you can always add a Vim requisite or extra magnitudes, or make the guideline InVi.
As for vis replenishment, I am not sure. They could try to ask the plant (InHe lvl 15, speak with a plant), it might actually know. Otherwise, I would say start with the lvl 4 InVi guideline "judge the amount of vis present". Figure out the Duration necessary for one pawn of vis to replenish. An InVi spell with that guideline and that duration would actually detect the replenishment (because it would detect different amounts at the start and end of the duration). Now add the equivalent magnitudes to the guideline. For instance, if it took one year for one pawn of vis to replenish, that would be +4 magnitudes, and you have the InVi 20 guideline "detect vis replenishment if it has a rate of 1pawn/year or greater", which can then be used with Momentary duration. InVi 15 would be "detect vis replenishment if it has a rate of 1 pawn/month or greater", and so on. InVi 17 might be "detect vis replenishment if it has a rate of 1 pawn/season or greater".
If one of them has a decent InVi, he could just spont the effect, and his actual casting total (minus a magnitude for Touch range) will tell you how small a replenishment he can detect. Otherwise, if they are going to spend a season inventing the spell, they might as well go back to my first thought and just examine the plant at regular interval to determine how much vis it contains, and after a season they should have the answer anyway.
I think that I would go for the InHe 3, learn a single specific fact about the plant would be better (to answer both the killing and replenishment questions). Possibly with a Vi requisite.
If you look at other InFo Guidelines, the low-level Base effects are all about mundane facts - for magical details, it usually bumps it to Base 10-15 or so (the few times it mentions such). But Guidelines are implied to be parallel for such things.
So I'd go that way - a simple spell can tell whether the fruit of a mundane plant is safe to eat, not the repercussions of monkeying with the magical aspects of a magical plant, which takes a little more. ims.
But yes - a spell should do it, even if it's not one that can be spont'ed by every apprentice.
For my two cents: That's what Magic Lore does, at least in my saga. InFo spells would help, but for things like that I require a season's observation and an (actually fairly trivial - the time's the important thing) Intelligence + Magic Lore check. This doesn't consume labtime unless the source in question is a long way away.