When can we apply for our citizenship?

It all depends on how much time you spent outside the US between 2009 and 2011. If more than 6 months at a time, you will need to show proof that you did not abandon your permanent resident status. If you fail to provide this proof, your status may be treated as "abandoned" and instead of a citizenship, you could be put in removal proceedings. But this is an extreme case scenario. Consult with an immigration attorney if in doubt.

You have to have 5 years of being permanent residents of which at least one half of that must have been spent physically in the US.