During Ender's training in one of his letters to Valentine he says that he has been training for three months with his launchie group (at the point only three of them passed the gravitation planet test). After that initial period we see he is assigned to Salamander Army. Now, the way the movie presents it, it almost seems like Ender is in 1 batte with Salamander Army before being promoted to commander. I think it is fair to assume that he was actually in the army for more than one battle, particularly since there was training time leading up to the battle which was portrayed.
After he is given his command is when we see Graff and the display of ships. And we to assume Ender clearly fought several armies as the leader of Dragon Army, before defeating the Salamander and Rat army in the 2 on 1 fight. I read that display to mean the human fleet was going to reach the Formic System in 28 days. Then he gets promoted to command school. However, before that he says he won't go and is back on Earth at the lake. He has time to build his raft. Maybe a week there.
Then he goes into Cryo-sleep on his journey to command school, seems to take 20 days or so? There is a display on his sleep chamber, and I remember thinking there weren't very many days left.
Then there are a series of battles in command school with the simulator and his team, before the final battle. These clearly take place over a period of days, if not weeks.
So my rough estimate is
- 3 months as a launchie
- 1+ month in Salamander Army
- 1+ week as commander of Dragon Army
- 1+ week at the lake
- 20+ days of travel to Command School
- 3+ days at Command School
This brings a low estimate to possibly 5 months and 1 week. It also throws off whatever the display Graff was looking at for the IFF Fleet reaching Formic space. Maybe there was a danger that the Formics would detect the fleet and demolish them before they had a commander? That apparently didn't happen.
As is the consensus of many, putting an extra five minutes into 5-6 different things would have greatly improved the development of elements in the film.