I think if you send a PM after 6 months or so, then give them a fortnight to reply to it. If they don't, then it's probably wise to delete it.

We should put this somewhere in the FAQ so that anyone planning on staying long without posting will be warned.

(Playing blind Pictionary, me drawing)
Ella: Is it a giraffe?
Me (stops drawing): No
Ella: Star Trek?
Me: Yes!