The ideal length for a movie?

It depends what the movie is. I generally prefer movies to be only 80 to 90 minutes long. If it's something like Superman: The Movie then I don't mind if it's longer than two hours.

These days, with my attention span, 90 minutes.

An hour and fifty minutes generally seems to be the length of my favourite films.

But films vary in the type of film they are and the type of length they need to have to fully breathe and let their story develop an organic life. A film like Once Upon a Time in America certainly would have lost something if it was cut down to half its length. Ditto for Betty Blue.

Sometimes though long films are allowed to get too indulgent by directors who need to learn when to become the required ruthless editor prepared to kill their own darlings. And there are always going to be those films today that may have two and a half hours of length but certainly don't have two and a half hours of decent content.