How I can make all downloaded feed items saved on my pc forever and not delete after certain time or size ?
if you disable the Clean-Up options under
Tools -> options -> Feeds -> Clean-Up
all data will remain in the database (feeds.db).
Please be aware that the db size will increase over a long duration and that might slow down QuiteRss.
Alternatively you can create a backup of the file feeds.db on a regular base before the news are deleted.
This backup should include the file QuiteRss.ini holding the settings.