I think one day is not enough and would be VERY exhausting. Sites are quite far from each other and if you like photography and just "being" in such a wonderful place, then take the time. Sometimes there are crowds and lines to climb up to the sites too. We used 2 days but if I had to do it again, I would have spent 3 days there. But we visited from 10am- 4pm only, (it was rainy season) and used our book guide instead of a paying one. Out tuk tuk driver also gave lots of infos. If you go all the way to Cambodia, then enjoy it and avoid doing the "been there, done that" trip.
It is $20 per day to enter the park and the sites or a 3 day pass for $40. You can get it at the entrance and they take your picture right there and it is printed on the pass. Passes are controlled at each site entrance.
Hvhighexp is correct. If you aren't interested in gaining a deep understanding of everything, then one day is enough to see all the main areas if you spend the whole day there. But if you want a guide to explain the history, symbolism, etc, then two-three days would be needed.
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