I did this for my campaign, so for what it is worth, here are some of the things I learned.
Apprentices are very weak, especially if they begin as children. Their characteristics are mostly negative, they have few Abilities, 0 in most of their Arts, no spells, and even their Virtues and Flaws might not all have appeared yet. Their small Size and negative Soak means they die easily. This is a challenge for GMs because you need to come up with stories they can handle. Conflict from non-physical sources is ideal. When physical dangers do present themselves, it needs to be the sort of thing that can be handled with spontaneous magic of level 5 or less. The APPRENTICES book has some advice on this.
But the weak PCs is also frustrating for the players, who signed on to play wizards. I advise moving through apprenticeship rapidly. I moved 1 year per game session, with a couple of two-parters. That was a 16 session saga. It was very rewarding. We all had a lot of fun. But it was a challenge.
Remember that, at least from an in-character perspective, apprentices make almost no choices about the magic they learn, the Arts they're taught, and the spells they end up knowing. This is determined by their master. There are a few ways you can go here. I wrote up all the masters, who decided what they'd teach, but the apprentices always had at least one free season of the year to read books on their own. I also gave them 1 point worth of "Cantations", spells of level 5 or less, for free every season. So by the second year they had spells of their own choosing, even if those spells were only level 5.
PC apprentices are going to be better than apprentices generated according to RAW. This isn't a bad thing, just be prepared for it and be ready to explain it. Apprentices will get more than 30 XP a year; their masters may have better Teaching, they may get access to the library (where a single Root of the Arts can give you 21 XP in a single Art in a single season), and they'll know more spells. (Eventually. Spells are usually taught late, because you need Arts to learn them. I did 3 rounds of spell-teaching over the course of the apprenticeship, one in year 7, one in year 14, and one in between.) If you use NPC masters, PCs will develop at very different rates and some will be happier than others. Masters may not have the spells and Arts which students want. If they're getting bucket loads of XP and spells, it may not matter so much that they're not getting what they want. Your Mileage May Vary.
An alternative is to simply not write up the masters at all, and presume that whatever the PC wants his character to learn, the master knows. This is much easier, and you can keep the XP under control. PC apprentices look more like NPC apprentices, getting 30 XP a year and 120 levels of spells, and that's it. Much less preparation time, simpler, but less true to the simulationist nature of Ars Magica and the PCs will be less powerful when it's over. There's no bad decision here, just different outcomes.
A special note on House Tytalus: Apprentices of House Tytalus are basically tortured for fifteen years. They're also usually children. This can be very unfun, and even morally wrong. It's not easy to just hand wave away the Book of Instruction when the RAW says every master uses it, even the ones who swear they won't. If no player wants to be Tytalus, you have no problem. If someone does want to play Tytalus, talk to the player about the Book of Instruction and figure out, between the two of you, what you both want to do.
For what it's worth, my campaign has extensive online resources, including a complete narrative of the 15 years of apprenticeship, with game notes and NPC stats.
sites.google.com/site/hboarsmagica/
) aren't bad or deluded (at least regarding Covenants.)