Your idea of the player ending the avoidance after the 50th song (read: 25 minutes) is absurd. There's a few inherent flaws with long avoidances:
-Attack variety
-Keeping interest
-Trying to keep it fair/balanced
For a 50 minute long avoidance, I have no idea how you're going to create attacks that aren't copied everywhere ... unless you're super creative, this is a big hurdle.
On top of that, keeping the interest of the player is key. Even for those that don't like avoidance, I know many of them will give it a shot if the fight seems interesting enough, well-designed, etc. Insta-gib attacks, especially in a long avoidance, is going to turn off a lot of the players that even reach this part of your game.
In the off chance that you actually create a full 50 minute avoidance with interesting attacks, great visuals, etc. etc., keeping it balanced is an entirely different monster that you'll have to tackle. Having the 3rd attack be monumentally harder than the 1st/2nd or any subsequent attacks is easy to accidentally do (which you don't want), which also makes attack variety difficult. It's a vicious cycle.
If you are still seriously considering a long avoidance (anything over 10 minutes), here's what I would recommend:
-Giving the player an option to have hit points. You said you were against it, but it really does make a lot of long avoidances fun, to some extent. Of course, the player can opt to only have 1 hitpoint (like "impossible" mode or something), but give not-as-skilled (or not-as-willing-to-grind-for-days) players the chance to have 10, 20, etc. hitpoints.
-Checkpoints are another option here. Maybe every 10 songs (that's every 5 minutes, according to your song length) have a checkpoint for the player, so that if he/she dies, it just puts them there instead of back at the beginning of the song. You could also vary this by "difficulty" that the player selects. Maybe at the 'impossible' difficulty, there are no checkpoints! But at the normal difficulty, there's 5 checkpoints.
-Don't follow a traditional avoidance route. Mix it up! There's plenty of really neat, interesting, and fun avoidances out there that don't just have the player dodging apples. Sometimes you have to do platforming, sometimes you have to perform an action, a gimmick, etc. This really keeps it interesting, and I would most certainly recommend something unique besides just plain "dodge this attack" avoidance.
Try to consider these suggestions. I hope this doesn't fall on deaf ears. A super-long avoidance is a neat concept, but you need to make it fun, engaging, and worth the time. Trying to invite as many people as possible to try your avoidance is much better than trying to overdo it and excluding essentially everyone except hardcore avoidance players. Don't become the "Happil 2" of avoidances.
Good luck.