Every 24 hours after the feeds sidebar was opened.
Changing a subscription “Keep Entries for” from 1 week to 1 day does not trigger an immediate cleanup. If you want an immediate purge after making a change to “Keep Entries for”, you can close the feeds sidebar application, and reopen it, the items will be purged before anything is displayed.
The new line in the title is causing problem you are seeing. The underlying Notes Java API used by the feed reader to look up existing entries must have regressed since 8.02 which is causing this problem.
Also with Lotus notes 8.5.1 not working keep entries function with rss feed reader. All Entries will keep forever items and changing time for keep entries not help.