No. Your journal entries are kept forever unless you choose to delete them. By default, only your last 20 journal entries or so are displayed on your main journal page, but you can change that. You can always access all your old journal entries from the Calendar page.
To lighten the load on LiveJournal's servers, only the last 75 entries can be displayed using your Recent Events view. If you wish to read older entries, you can access them through the Calendar page.
