@sf9 Great question!
On IcePets, we don't have any breeding or anything, so they effectively only hunger/get bored on days you show up at the site, that said they do age. But that's when it gets complicated in your situation.
As bad as it may sound, I think "the show must go on" and more importantly it's the user's decision for not showing up on the site, would trigger me to essentially say if there are core mechanics that work with the site, your first goal is to make those stable. Unforunately it does punish some users who disappear for a while and then come back. That being said, the likelihood of returning after being gone for 6ish months is already pretty low IMO. It becomes back of mind to them. So, it's best to almost treat them as a churned user and then move on. If they come back, you could always offer a "welcome back" bundle which could help restore some aspects of their pets in order to re-incentivize.