Ok, I just completed this today and it took just under 8 hours. Its pretty easy but time consuming to get. You can use any instruments, even switch if you like, I left my keys on pro, and my mic plugged in incase I pressed the wrong thing. Because I'm pretty sure 6 hours in, if I messed it up I may have gone on a rampage! Ok, so that in mind, simply start the endless setlist 3, play like any setlist. You don't need to get a certain score even any score for a song so if you are feeling cheeky you can just let the song play by itself and move on. I did this with a couple that I hate playing. But its a good opportunity to work towards other trophies, I also got the silver pro keys trophy while doing this.
So basically keep playing the setlist, all in one go, you cant save or pause so make sure you set aside an epic 7 or 8 hours then in the end this beautiful gold shiny trophy will be yours :)