Do you have newsletters on your forum? What type of content are included?
I send a monthly newsletter on my forum out by PM and email usually around the last week of each month. It has things like news, updates, contests, interesting discussions, etc.
Newsletters are great as they remind your inactive members that you still exist and might tempt them to return. However, I only send newsletters when there is value, if nothing interesting has happened then I don't bother as people may just regard it as spam. As a result, the newsletter may be once or month or once every three months, it varies a lot.