I'd broadly agree with the above in relation to heritage railway Guards training.
Generally speaking it's done 'on the job', so as has been said it'll depend on how much time you can commit (and how much trainer and assessor availability there is at any given time) as to how long it would be before you "pass out" as competent in your duties.
On the job training usually consists of going out on a diagram with a trainer guard who will go through the training requirements with you out in the real world. There might be classroom elements and occasional courses off trains, depending on the railway in question (sorry, can't speak for the Bluebell).
You'll generally have to learn about safely accessing the tracks, learn the locations of different features on the line (eg Stations, viaducts, tunnels, signals etc), gain a familiarity with the rolling stock (eg what each bit does below the solebar, any internal equipment, things related to coupling vehicles together, components of each type of braking system), understand the emergency procedures and how to use the kit, learn the individual railway's operating procedures, and the procedures for ensuring the safe departure of your train from stations.
You'll probably also have to learn how to shunt trains (which is where you're coordinating movements from the ground, or the far end of a reversing train), and depending on the railway you might have to do tickets too.
It's generally interesting to do and whilst often a bit repetitive every so often you'll get that 'something different' happen that will challenge you and make you use your brain a bit.