Soon after communications are cut with Mission Control, Kowalski has Dr. Stone (Sandra Bullock) set a timer for 90 minutes, having figured out how much time it will take for the satellite debris that destroyed their shuttle to round the Earth and wreck havoc again.
That hour and a half is not arbitrary. The real International Space Station takes about 90 minutes to orbit the planet, traveling at 17,500 miles per hour (not the "50,000 miles per hour" Kowalski uses to make his calculations).
"Gravity" has a running time of — you guessed it — 90 minutes. Thus, in the time that audiences experience the movie, the space station flying 240 miles (390 kilometers) above has completed one trip around the Earth. (It's not uncommon for the astronauts and cosmonauts aboard the orbiting outpost to watch movies on DVD; if Warner Bros. Pictures' arranges to send "Gravity" to the station, it could make for a novel way to spend an orbit.)
Based on the number of times Dr. Stone encounters the satellite debris cloud, it seems she completes three trips around the planet after being separated from the shuttle.
