This achievement isn’t hard, just can be time consuming. It greatly depends on difficulty and how good you are. If you play on expert and can hit a majority of the notes you could unlock this very quickly. But if you play hard and struggle a lot, this might take a lot longer. My suggestion is if by the second playthrough Quest you still haven’t unlocked this is to just keep playing songs you like, or songs that have lots of notes that you know you can hit.
I have taken a guess at around 750 notes hit for the average expert guitar player, this means 200 songs are needed if you are averaging the 750 notes hit.