Let me preface this post by saying I have so far cancelled two trips to REP, but will get there one of these days

My research indicates that, as has been aluded to earlier, it depends on how much you like temples and history. Some find one day plenty, some feel that four is better. There are some locations other than just Siem Reap that are further away and have less tourists, so will probably take a full day for each of those @:-)