From what I understand, there is no set rate at which one can post ... If you want to start a new topic that is fine, it's what keeps these groups going to be quite honest, however .... if there are threads already on the go and there usually are, yeah it is generally polite to respond, if it seems the type of thread you would respond too ... basically if someone is expressing themselves to the other members of this group it would be nice to let them know that you hear/read what they have to say ... technically though, there is no "rule" on this ....
I know for myself, I start at a day before thread and work my way up to the new ones ... ie: 3/9/2007 with today being 3/10/2007 ... that way I don't miss any "new" messages, but that is "my" preference. Others may just decide they saw enough of one thread or another and move on ...
I think the best way to avoid this is to expect the unexpected OR if you would like a response, let the group know ... ie: if your looking for advice etc ...
hope that answers your question!