Was on Netflix but sadly not right now. Hope it's back on soon.

From Ebert's review:

Every review of "The Best of Youth" begins with the information that it is six hours long. No good movie is too long, just as no bad movie is short enough. I dropped outside of time and was carried along by the narrative flow; when the film was over, I had no particular desire to leave the theater, and would happily have stayed another three hours. The two-hour limit on most films makes them essentially short stories. "The Best of Youth" is a novel.
. . .
The film is being shown in two parts, three hours each, with separate admissions. You don't have to see both parts on the same day, but you may want to. It is a luxury to be enveloped in a good film, and to know there's a lot more of it -- that it is not moving inexorably toward an ending you can anticipate, but moving indefinitely into a future that is free to be shaped in surprising ways. When you hear that it is six hours long, reflect that it is therefore also six hours deep.

SF Gate's LaSalle:

A six-hour movie is a challenge, but a great six-hour movie is also a gift. "The Best of Youth" -- an Italian film covering 37 years of political and social history through the story of one family -- is so in-depth, so appealing, so easy to sit through and so anomalously grand scale that few who see it will ever forget it. If it were a novel, it would be 1,000 pages long and a page-turner. It's epic. It's a full meal in a world of snacks.

CS Monitor:

This is epic filmmaking on a profoundly human scale, directed to perfection and magnificently acted by everyone in sight. Not to be missed by anyone who cares about the joys, sorrows, and challenges of living in the modern world.

AO Scott:

The story he has to tell, written by Sandro Petraglia and Stefano Rulli, is full of nuance and complexity, but it is also as accessible and engrossing as a grand 19th-century novel. Yes, "The Best of Youth" is long. But "War and Peace" is long. "Middlemarch" is long. Life is also long, and there is so much life in these six hours - 37 years, to the extent that you can quantify it - that you may marvel at Mr. Giordana's economy.