There are not really guidelines it really depends on your audience and the quality and targeting of your messages. Also, it helps if you are setting frequence expectations for your audience and not emailing too many "acquired" lists.
I know some marketers that send ~30 messages over 30 days to their audience (but they are highly targeted workflows). I know others that are just email a 2-4 per months. The thing is you hear about people in both scenarios getting black listed if they are using poor quality lists.
Keep a close eye on your unsubscribe and deliverability rates. Also you should monitor your standing with the blacklisting services.
http://mxtoolbox.com/blacklists.aspx
Hope that helps,
Jeff