The longevity of news feed items is actually hard coded to 7 days and it is cleaned up periodically. Every hour, in fact, we will remove any entries from the xf_news_feed table which are older than 7 days.
So for you to have less than 7 days may mean that the data has been emptied from there manually, or there is some other process which has removed this. Uninstalling an add-on may touch this table, but obviously add-ons would be aiming to only remove news feed entries they created, but sometimes mistakes happen.
I guess we should check to see how old the oldest item is in that table, could you run this database query?
SELECT news_feed_id, event_date
ORDER BY event_date
This should give you the news_feed_id and the timestamp of the oldest item in the table.
If you can't do it, you may need to consult your host.
Database queries can either be run from your server's shell or by using a web based tool like PhpMyAdmin. Most hosts make PhpMyAdmin available, especially if you use something like cPanel for your hosting.