I have noticed that the weekends are the most populated times.
The american servers are the only ones with a big population and times when people here in the usa are not working/going to school are when you will find more people online.
So I would say besides weekends, after 3pm est daily until midnight-ish pst.
That being said, unless there is a zaishen quest, most outposts will be pretty barren of people due to folks playing with heroes mostly. You will find people hanging out in kamadan mostly spamming wtb/wts stuff (and afk---). LA has a population at times, I rarely see people in Kaineng for very long. Other place folks can be found is the great temple of balthazar (during the peak hours listed above).
where is the 'all you can eat' cookie bar?