I must admit, it is really difficult to play multiplayer lately, for several
reasons (i.e. not being able to host/not knowing how to open ports).
There aren't really certain times when people are going to play, but you
will have more luck if you try it on the weekends.
The IRC Channel (#siedler2.5 on freenode or the "Chat" link on the homepage
which leads there) is also a good place to find people to play with.
Or if you use Steam on a regular basis, you could also join the group and
ask people who are online if they want to play. There are about 25 people in
the group at the moment, so it is likely you will find someone to play with
sooner or later. You'll find the link in my signature.