Gravity is pretty short but still brilliant and powerful. No need for every blockbuster to be 2/2.5 hours like most are today.
Not sure Prometheus is a good comparison, more Life of Pi angle.
Just saw Gravity at a preview screening.
Absolutely breathtaking brilliance encompassed and amazing in 3D on the big screen. Would be criminal if this did not pass £20M at least. Best film of the year so far.