You left out some major information. If you are from Somalia or North Korea you are going to be waitng a looong time.
However, the Cambodian Visa can be done on arrival at the border if you are coming from Thailand. Many provate tour companies will do this process for you. I will warn you now, many tour companies will charge you extra money for the visa and lie to you about the price. You can do it all yourself and you do not need them. But unless you speak Khmer it may be difficult and you might get tricked into paying more to the customs official. The processing time with a transportation company usually takes 2 hours and another 30 minutes at the border if traveling by land. In the airport it should take 30 min if there is not a lot of traffic when you arrive. It is $30 in the airport. Cambodia uses USD.
If you are flying to Cambodia and are a US citizen I would just do it when I get there. However, you should check with your airline as some airlines will not allow you to even fly unless you already have your visa and proof of onward travel out of the country (flight to Vietnam, bus to Laos, etc). It depends on your citizenship, the airline, and your travel plans.
I knows this from visitng Cambodia 3 different times. But, I must note, the rules often change with time and depending on what customs official you get.