The average time it would take to 100% would probably be around 50 to 60ish hours. So it's a good idea to check your Wall of Fame on what you are missing. Most of them are finding stuff or just doing the same action for X amount of times. One of the achievements require you to complete the Wall of Fame (The big board at the hangout.) It's basically your in-game achievements that give you rewards and some of these are tied to steam achievements. (One of them is a unlock which requires you to beat the game.) Also I used Easy difficulty for getting those no-hit boss runs, but you can play any difficulty you desire since later you'll unlock level select.

I recommend starting on either Normal or Hard if you can't get used to the feeling right away and you will get upgrades later so it makes the other difficulties easier. This game has no missable achievements since you can just select the levels again, though it does require multiple playthroughs because of the difficulties and requires you to backtrack anyway for other missed content.

Hi-fi rush combos