I visited SR and Angkor complex last month.
I had planned to spend 1 full day in Angkor to visit the main temples but the response I got on the forum to my suggested Cambodia itinerary was that I would need 2-3 days minimum.
In my opinion this is not true, unless you are very, very interested in temples.
I went to Angkor Wat for sunrise, Angkor Thom complex, Ta Phrom, travelled to Banteay Srey, visited a shooting range, returned back to Preah Khan and Angkor Wat all on the same day. I returned to hotel between 3-4 pm.
I did not rush my visits to any of the temples and stopped for lunch, drink stops etc. I felt that I saw the "highlight" temples and chose not to buy another day pass for Day 2 to visit the less "rated" temples.
If you have limited time in Cambodia or SR, I feel that 1 day is plenty. I do not understand how a typical western tourist could spend 3+ days visiting the main temples. If you want to visit the temples at a more leisurely pace 2 day would be adequate.
Since I had booked a driver for 2 days, I visited Tonle Sap and the floating villages which I enjoyed.