You can only bump your Request Thread(s) once per 23 hours.
You can make posts to threads or PMs as often as you like, so long as you're not violating the bumping rule (noted above). There may be a server limit to the frequency of posting, such as one per minute or 30 seconds or something, but that would be to limit bot/spam activity and shouldn't impact people too much.
there is a server limit on new accounts for conversations, it either goes away with time or with post count, I can’t remember. it isn’t forever, though! AFAIK it’s a way to make sure new accounts aren’t spamming conversations.