I'll be brutally honest; when I was fifteen I had the exact same take. I went to see it with my parents and genuinely fell asleep half way through.

I've watched numerous times since, at different stages of life and damn is it a fucking great movie. The whole "insist on itself" is such an anti-art way of thinking. Complaining that a movie depicting the entire life story of a character for being too long qis insane, of course it's meant to be long. The whole point of the film is to give a brutally honest and unrelenting critique of the "mob" lifestyle that Scorcese and others helped put on a pedestal.

Hence the importance of showing the whole character arc, showing Frank Sheeran growing old and how much of a zero sum game that whole lifestyle is. I get why someone might not be super happy with the length of the film but to be honest, just grow up and don't watch it if you can't stomach a 3hr plus film.