there's a useful discussion on how much Cambodia costs already going on over here: http://answers.worldnomads.com/questions/cambodia/3907/how-much-would-it-cost-to-backpack-in-cambodia-for-one-month
As for time and activities, it depends how you like to travel. I spent 3 weeks and could easily have done with another one - went to Siem Reap, Phnom Pehn, Sihanoukville - each one was a base and we did day trips in and around those regions. Angkor Wat is a must-see, but if you're more adventurous, also grab a driver and head out to see some of the more remote temples like Beng Melea: http://en.wikipedia.org/wiki/Beng_Mealea
I also really enjoyed the time we spent down on the beaches. It's easy to rent a motorbike and leave the tourists behind, not to mention the great seafood we ate in beach cafes with our toes in the sand.
If you look for other questions on Cambodia on this site, you'll find a lot of recommendations from other travellers too about 8 years ago
I did 2 weeks in Siem Reap and felt that this was enough time to do justice to all the temples, the markets, the restaurants, the people etc. I could easily have spent longer and gone on the Mekong to PP as well as to the beaches etc.
Cambodia is amazing and deserves some time. almost 8 years ago