I've put something between 8 and 10 hours into the journey mode of the game. I guess it's analogous to the career modes in other music games. The first "gig" that you prepare for is two songs if I remember right. The first of which is Satisfaction by the Stones. It starts you off with a few core notes of the main riff. For me, by the end of the first play through it had upgraded my skill on that particular phrase to the complete thing. A few of the other spots are similar to that. As you go through those and the game learns how well you're doing, it starts the songs with more notes in them, and yes, after a little while you are playing the entire song.
Also, if you already know how to play the song, it doesn't penalize you for playing more than it's giving you. It simply makes sure that the notes on the screen are being played accurately. As an example, there's a drop-D Soundgarden tune on there that I was having trouble playing just the on-screen notes for. So I started playing it fully because I know how it goes, and shortly afterward Rocksmith started giving me the full pattern. It's actually a pretty ingenious engine. I'm having fun playing with it because I don't generally play that much guitar, so it's helping me improve by learning new songs, and some note patterns that I likely wouldn't try if I was just noodling around on my own.