what are good post-intervals for ranks

The problem is that it isn't necessarily dependant upon how popular a forum is, but rather based upon how active your members are. If your topic is of a nature where most members are very active and constantly engaging in thought-provoking discussion and conversations, then that is different from a support site where most people only post questions when they have a problem with something.

When you're starting out, I would start with lower numbers, because that encourages members to continue posting to the site, and is a good way to get a new forum off the ground.

i decided not to do any ranks/titles for the time being. maybe i'll start a thread on the topic later on. i think a lot of the people who are using it are used to email lists and they might not like being called a n00b, newbie, or apprentice.

plus i've heard the argument that it's quality, not quantity that counts, and phpBB doesn't have a reputation system built in.

Quality over quantity is definitely the way to go. Good luck.