I believe that the value of the non-lab assistance work an apprentice does is why the assistance provided by an apprentice is more valuable than ezzelino apparently thinks it is.
The most valuable service that an apprentice provides over the course of their apprenticeship is the copying of lab texts and casting tablets. After a season of tutoring in profession scribe your apprentice turns into a valuable source of texts and tablets that can justify their one season of teaching for three seasons of copying.
The apprentice can fix permanent arcane connections, This can be done immediately after opening the arts it is something that every nearly magus has a use for and it is something that almost no magus wants to spend a season doing. Thus there is a market for it with lots of buyers, it seems that three seasons of this might justify one season of your magus' time.
The apprentice can extract vis it isn't much but it on;y needs to be worth a third of the value of a magus season to be worthwhile.
The apprentice gives you the opportunity to get vis and and texts it makes sense that they can use lab texts to craft low level lesser devices and charged items especially near the end of apprenticeship.
The laboratory assistance provided by the apprentice is only valuable when it makes the difference between saving a season or being able to do something that you otherwise couldn't do. this isn't particularly common for any individual magus but it seems sensible to "rent" your apprentice to other magi who do need his or her assistance saving someone a whole season or allow them to achieve something that they otherwise couldn't is probably the most valuable service that the apprentice can provide on a per-season basis.
The first two pages of this old thread really look in to how apprenticeship time would be spent Napkin scrawl apprenticeship time distribution