I have never seen the "1 season begrudgingly given, and the other 3 seasons the apprentice being a lab slave" happen in actual play. Even if a given Magus had the personality for it, most do not spend three seasons in each year doing something their apprentice could actually give them a bonus in. Then there is the lack of age and skills found in most apprentices early on, where they are very young (with poor Attributes due to age modifiers) and completely lacking in any abilities.
So they need to learn Latin, both speaking and reading/writing, along with basic education and magic theory. Even if the Magus does not teach those personally, the seasons are still taken up by the learning. For the first several years that will easily eat up at least half of an apprentices seasons. When you take into account all of the things they have to learn, with not hitting the minimum in any bad so most will slightly overshoot at the least, along with the fact that the apprentice will often actually be a lab negative early on you really have maybe 20~30 seasons max "lab slave".
You do not take an apprentice for a lab boost. With the 15 seasons of teaching plus however long it takes to find them a Magi could get a better lab boost by studying or enchanting. You take on an apprentice because they are a form of legacy.
From my personal experience, apprentices play generated often end up over powered. Even if you try to do the minimum they will end up more powerful then a book generated Magi. And if the Magi tries to push the limits they will end up horribly over powered. Like near game breaking. 21 year old just past gauntlet Magus that is more powerful then any Magus many on the forums have ever had in a game.