All the time needed to master a game like that ... you could learn to play a real guitar.
(I'm betting the people who master those things can't actually play a real one)
Well, but if your thing isn't playing a real guitar and you just want to have fun playing video games?
I do believe the cost is less also... for you to buy a guitar and amplifier it is probably more expensive than a game + controller (assuming you've already got the console). Not that it matters though, it all depends in what you want to invest your time / money into...
I have played guitar hero a lot... it's not quite the same as this, that is certainly a simpler controller / concept, but it was lots of fun having friends coming over and playing together... and I didn't need all of them to know how to play an instrument and have gear etc etc...
It's fun, might not be everyone's cup of tea of course, but others find it to be fun... I'm pretty sure many people don't understand why we bother messing with solder and PCB's when you can just buy most stuff from China but hey, it's a hobby right?