It has been pointed out repeatedly in the past that, according to the ArM5 rules, apprentices often are not worth the time spent educating them (not to mention the effort to find them in the first place). Thus, the idea has emerged that you take an apprentice not for the help provided, but as a sort of "adopted child" to create a legacy.
I think this is a problem with the mechanics, and specifically with the ArM5 mechanics, where apprentices are somewhat less useful, and the opportunity cost of teaching them is comparatively larger, than in previous editions. Instead, I think the rules should support the notion that an apprentice is to a magus very much what a modern PhD student is to his advisor: an asset, that requires time and material resources to develop and put to good use, but that if correctly supervised turns out to be a decent-to-good investment before graduation. Note that this does not prevent a bond of parental-filial affection forming! But even to be considered good legacy material, a PhD student must pass this "net worth" test.
Furthermore, I think the rules should do two more things. First, an apprentice should boost the master's productivity not by allowing him to do tasks he wouldn't be able to do alone (so, no bonus to Lab totals!), but by allowing him to do more of the same stuff - basically the master offloads lots of chores to the apprentice. Also, a selfish master who optimizes his teaching to extract as much value as possible from the apprentice, should still more or less produce a "standard" magus. Contrast this with ArM5 rules where the master would be in many cases better off telling the student to study on his own from books or from mundane teachers, and in any case would be better off teaching the student Magic Theory rather than Arts or spells.
Mechanically, I'd do it as follows. Crucially, this is about apprentices, not collaboration with other magi or your own familiar. I think those should be handled in a different way (in particular, I am not overfond of familiars adding their Int+Magic Theory scores; instead, a familiar should aid a magus using mechanics similar to those of Laboratory Personalization found in Covenants).
Hermetic Apprentices
If you have an apprentice at hand, every two consecutive seasons of magic work you gain a third one for free. A season of magic work, for these purposes, is one where you'd only gain exposure experience, and this experience could be in Magic Theory. This represents the extra productivity you gain from a second pair of hands, and assumes you do spend some of your time instructing your apprentice. In particular, for every season in your service the apprentice gains:
- 4xp in Arts or Abilities, as long as his score does not exceed yours, and
- 2 Levels of spells, that you can "spend" at any time during the apprenticeship to teach him spells you know of Level no higher than his Tech + Form + Int + Magic Theory +3.
You can actually skimp on this instruction and squeeze as much work from your apprentice for as little instruction as possible, in which case every three consecutive seasons of magic work you gain two more for free (in other words, you get 5 work seasons every 3, rather than 3 every 2), but then the apprentice only gains the "standard" 2xp of seasonal exposure experience for helping you. This is considered niggardly and exploitative by most magi (including those of House Tytalus) and is a low crime in many Tribunals and a source of Poor Reputation everywhere.