I think 6 months is too long. If no-one has posted after 3 months, there probably not going to ever bother. I say after 3 months send them an e-mail (not PM) saying there account as been made inactive. The only problem I have with this is that who can be arsed going through the list every 3 months (went we get bigger it will be more often) e-mailing and deteling accounts.
If mal is the only one who can delete accounts it's alot of work for him. An unfair about of work.