It's not difficult but I think an afternoon is an unrealistic timespan. I learned in a three month module of a journalism degree I ended up dropping out of and this was the textbook (from 1996).
It's one of those things that's easy to learn but hard to master. If you put your mind to it you can learn the motions in an afternoon, but to become proficient at it requires a lot of practice. As >>5857 says, a couple of months of proper practice is enough, but any less and you'll be no faster and prone to error. It's a mechanical skill, really, and that requires muscle memory; if you're good at that maybe you need less time but please please please be aware of the difference when it comes to knowing how to do something and being able to do something.
If I wake up and don't have to get up properly for a while, I daub the shapes of song lyrics in teeline. It's just a weird habit I picked up but it got me a lot better at sight-reading it (rather than having to decipher it).
My mum said her shorthand tutor advised her to practice all the time, e.g. every time she sees a sign, to visualise it in shorthand.