How can I configure how long unread items are kept?

I’m using Newsblur on Android with Premium subscription and I noticed that Newsblur keeps unread items for some feeds only a day or so whereas for others I have them for multiple weeks.

How can I configure retention?
And is it possible to configure how many unread items are kept (something like aFIFO queue for items)?

Thank you?

PS: I love Newsblur - great work!