So, I asked this question back in December of 2012, and Marko's response was:
They gain normal xp for whatever they did, so long as their magus was involved somehow. Familiars are not magi though, so the 10xp per season does not apply to them (nor does it for grogs or companions). The xp gained is based upon the value for the adventure they accompanied their magus on, exposure for lab assistance, and Teaching/Training based upon the scores of their magus.
The way that I read that and have interpreted it since then is that the magus has to be directly involved in any XP gained by their familiar. So, as I see it, familiars can gain XP...
- When they accompany their magus on an adventure (source quality of the adventure),
- When they assist their magus in the laboratory (exposure xp),
- when their magus (and only their magus) teaches or trains them (source quality based on magus)
That's it, nothing else. That's based on what Marko said back in 2012. The advantage is that it keeps us consistent with what we've been doing, and cuts back on the bookkeeping for familiars.