How much time needed in southeast asia?

How much time needed for southeast asia (vietnam, laos, cambodia, thailand, malaysia)? Anyone who has travelled to this area and/or have a suggestions on how long i need in each country? I know its a pretty wide question, but im going to be backpacking, and need a time frame. I have from one to four weeks approximately per place. Any suggestions to what to see? Thanks!

I think your estimate is right. Good luck!