Can't deny that the Sith guide is a bit vague on the numbers. Probably something we should grab Echo for sometime to fix up.

The best way to estimate is to look at the sort of skills a neophyte is expected to know before earning the rank of Apprentice, then extrapolate from that how long it'd probably take to learn it, while taking into account your character's aptitudes and age/maturation.

as for becoming a titled apprentice? That's (almost) entirely at the whim of a Sith's master. It can happen quickly or it might never happen. 

We've not read through the sheet itself again yet, but we will look at doing that later on.