That is a difficult question because it is unclear which army you are talking about. Is this referring to Ender's time at school, or Ender when he is unknowingly preparing to lead an army against the Bugger's?
The answer to the first question is just a few weeks. He was supposed to have a few months, but the leaders are trying to prepare him to deal with extreme odds.
With his reunited army at the International Fleet's command post, Ender has only a few short months to train before his big "final exam". This exam, of course, is the battle against the Bugger planet.
well to tell you the truth i never really says in the book how long they trained, but if i had to guess i would say they trainned for at least two hours, because of the context clues.