My rent boxes "evict" tenants when they're 5 days past due. I try to keep an eye on boxes before it gets to that point, but sometimes the eviction notice is my first indication that they're late. At that point I drop them an IM and copy/paste it into a notecard which I send them as well. In my message I say something to the effect of "If I don't hear from you within 24 hours I will reclaim the land." A little over half the time I get a frantic IM saying "OMG I forgot" and they pay the rent. But if I don't hear back I take the land.
I guess the key, for me, is communication. If you make it clear that you're about to reclaim the land and they don't act in time, you can't be faulted for taking it back. The rent boxes do send "rent due" and "rent late" notices to people, but they are green object IMs and they don't always get forwarded to email if they're offline, so I give folks the benefit of the doubt until I contact them personally.
EDIT: A couple days ago I was very sad to reclaim an openspace sim from a friend who's apparently vanished from
SL and
SLU. She was almost a month overdue, so I had to do it, but it made me very sad. I took pictures of her place before I returned her stuff.