How long does the alien invasion take in Bureau: XCOM Declassified? There seem to be many alien buildings constructed on Earth already when you attack them, so where did they find the time to build them? Some even appear to be labs, not just strongholds. Does the invasion take weeks, months or years?
The invasion is pretty quickly as the aliens are using the 'seeds'. When you play the intro level, there is a moment where you see a crater with a blue light in it. This is one of the seeds that impacted on the earth. Later you encounter it a few times more. The last time where it absorbs part of the train and the tracks.
Using this technology it is easy for them to pretty quickly create all the different buildings. It is also quite visible during the different missions that some of the alien teach grows very quickly. When the towers sprout out of the ground for example.
So that explains the quick construction of the alien buildings. Another easily missed thing, between the first mission, and the second mission, a few months passes. (Not sure how much, but one of the minor characters mentions something about it being a month). So that explains the apparent high speed of the invasion.
So no idea how long the invasion takes. But apart from the facility build by Axis, I don't think the aliens build anything before the invasion. They just have superiour building technology. (One of the scientists even remarks on this, that they will salvage the seed technology for the rebuilding process). Hope this answers your question.