You can unlock the final area with 32 cubes collected and I achieved that in about 6 hours of total play. I have many more cubes to collect and plan on doing that here and there over the next bit. The ending sequence was really interesting and I don't want to say much to spoil it but I will say that it was unique and thought provoking.
After you complete Fez you can create a new Fez+ game that starts the main campaign over again but allows you to pick up where you left off in terms of collecting things. I many areas left to explore and cubes to collect. Some of the puzzles in Fez are fairly complicated and I am not really sure how to complete them yet. That said, I have many areas left to explore and sometimes exploring an area tips you off to how to complete a series of puzzles you didn't have the key to earlier.
Fez is great and is currently part of the Humble Bundle 9 and I seriously recommend you play it.