At the ends of s01e02, the recruits are given a speech when joining the Survey Corps by Keith Shadis, and he says that they will be training for 3 years before they are expected to fight any Titans.
Why does it take 3 years to train a Survey Corp member? That seems like a good deal for all the refugees and should have caused their ranks to swell, especially with the now overflowing motivation of urgency. Three years of food and housing is nothing to sneeze at in those conditions.
For reference, most modern militaries have boot camps at most a few months long. US military boot camps range from 6 - 13 weeks depending on the branch of the service. Advanced non-infantry job training may take longer. Modern special forces may go through a number of schools after basic training, but still less than 3 years worth. Historically, boot camps were much shorter and less organized.
Three years seems like an especially long time for a para/military boot camp, though it does correspond to the length of time in attendance at a Japanese high school, which is also a common setting for anime. Is this more than a coincidence?