It should probably take you 4-5 days to beat Metroid, upon which you'll see your percent completed and go "Jesus, I have to play that again!"
I believe mine the first time through was 42%.
Tell me once you get to Serris; he's the first really annoying boss. The spider boss was the one that frustrated me to no end. After that, the really only annoying one was... well, I won't say, but if you've played any of the other Metroid games, you'll know him. Think wings.