This is really in answer to a question by tamtam in a thread in the Gay Thailand forum, but it's better that I put the answer in Gay Cambodia so that it will be easier to find in future.
I was there for four days last month, for a couple of work meetings and considerable free time. I've been to Phnom Penh perhaps ten years before and thus I didn't need to do the standard tourist attractions -- not that there are many. Two days, I think, would be enough. I would recommend the National Muse