Why? Taking a single country can take many months. Why would taking over an entire world not take longer?
Now, years, as in 5 or more, might be a little excessive, but I could easily see 1 or 2 years going by to root out all the defenders on a large, highly populated world. Unless you want that modeled by the rioting of the planet, but then all conquered planets should be rioting, not just the ones that were rioting before combat ended. Usually the first couple of enemy planets are jubilant or happy when I conquer them.
