From post here and a couple of brief mentions I've seen.
If you have one that you're meaning to keep and haven't posted for a long while, go put a status message or something up there.
How long is a long while?
To clarify, an inactive personal journal refers to any journal that has not been logged into for two consecutive years using any method of logging in, such as logging in while posting a comment, AND contains either no posts at all or only the LiveJournal welcome post. An inactive community refers to any community that has either no posted entries or only the LiveJournal welcome post. We also want to let you know that a journal purged for inactivity will NOT have its external content purged. This means that any comments that journal posted outside the journal and any community entries that journal made will remain visible even if the inactive journal is purged. After we finish purging suspended accounts, we'll be notifying inactive account holders to let them know if their journal or community is slated for purging. Users will be emailed three times over the course of six weeks. In addition, once an inactive account is deleted, the user will have 30 days to undelete.