Cambodia in 2–3 days
If you want to do Cambodia in a hurry, fly in and out of Siem Reap and spend your time at the Angkor Wat. There’s easily enough to fill in three days at the temples, but make sure you also visit the excellent Landmine Museum.
Cambodia in 7–9 days
Starting in Siem Reap, give yourself at least three days to explore the awe-inspiring Angkor temples.
There are several interesting temples outside the main Angkor Wat complex that are worth considering as a day trip from Siem Reap: Beng Mealea (an amazing abandoned temple); and Bantay Srei (a temple with elaborate carvings).
From Siem Reap, take a bus (about 6 hours) to Phnom Penh.
Give yourself at least one day in Phnom Penh to explore the Tuol Sleng Prison and the Killing Fields, and you could easily fill in a second day exploring the other sights of Phnom Penh.
From Phnom Penh, head down to the south coast of Cambodia (about 3-4 hours by bus) and check out Kep, Sihanoukville and Kampot.
Cambodia in 2 weeks+
From Siem Reap, catch a boat over to Battambang, for a town that’s more authentically Cambodian than the tourist driven Siem Reap.
From there you can head down to Phnom Penh.
You could also add in a trip to north-eastern Cambodia, which has some interesting places to visit (most notably the temples around Kompong Cham) and far fewer tourists than the rest of the country.