90 days seems too quick to me. A year could be too long too. Maybe 6 months? 

I have kind 1 and kind 30023 that I want to exist forever, but I also have paid relays and my own for those. 

 Yeah, 30023s are valuable 

 Authors of 30023 already pay for their relays because they know their post can just disappear if they don't.  

 Ah, but they don't if they're approaching habla the same way they would medium. You would assume your publishing platform keeps your content for you. But I am coming around to NOTIFY. 

 Agree, we need to work on that assumption. Updated UIs, NOTIFY and DMs can be helpful.