Unless you are an avid traveler looking at history, architecture, two nights are more than enough. just make sure the guides do not take you shopping. we just said we are done shopping and instead will tip him better for not going to the obligatory stops, and if nor " we will see" Luckinly steeped. As we are in the history of the temple building and correcting the guide on the mythology earned us some cred !

Do make sure to see the small. Useum of the Khmer atrocities etc. I think History or Discovery channel had a fantastic show on the Angkor Wat. Your library should have something.