If you periodically prune your neighbors...
If someone hasn't logged in for a while and they don't clear their towns, I keep an eye on how long they're gone for and if it's a week or more I will probably delete them, but it depends on the situa...
Forum Discussion
I'll delete them if they haven't been active for a few weeks unless they are forum members. That said I still have a few people(non forum) who have been on my list since I first started playing I had a few weeks where I couldn't play at all and they never deleted me
