The length of your trip of course depends on how many days do you have to travel. Here are some suggestions for those who come to Cambodia for the first time: Visit only Phnom penh: 3 – 4 days
The main places about Phnom Penh are old French-colonial buildings and historical sites.
Visit only Siem Reap : 4 - 5 days.The main places that you might want to experiment is Angkor Archeological zone.
Visit country in the most basic way: 7 -8 days. Within 7 days you will have enough time to go to main and popular cities of Cambodia like Phnom penh, Kampong Cham, Kampong Thom, Siem Reap, Angkor Temple, Tonle Sap .
Visit all country combined with beach relax: 14 days.Besides cover all main tourist historical attractions, you can extend the trip to some beautiful beaches: Kep or Sihanouville
Visit all country combined with some adventure and beach break: 14 days.Within 14days , you will cover main places: Phnom penh, Kampot, Sihanoukville, Battambang, Siem Reap , Angkor Temple