It took me at least 10-16 hours of gameplay to get mine to Virtuoso level @hallowchild. I didn't have great traits (just always friendly). I didn't use too many need potions although I did now and then.. mostly as I saw the end in sight lol. I sent my sim home to tend her garden and drop off bought/read tomes. I think you can learn it all by yourself or through the sages though.. I just didn't want to wait and besides reading the tomes looks darned good at a certain level. I sent my sim to festivals and did other gameplay now and then although she doesn't have a job, job.
I feel like I was still grinding way more than I normally would but since it was all new it didn't feel like a grind.
It's not a race. I plan on doing it even slower on the next spellcasters I make. Not to mention just living the lives of the top level casters as normal.
@RobinGreenthumb can you scrubaroo the kids? .. I'm thinking this could be fun for once. Then you can zap up a meal for them? I'm thinking of doing a black widow again with a spellcaster, love potion them up then have a baby with each to seal the deal before they meet with an untimely death, hehe.