I voted for six months. The only reason I give it that long is that sometimes we have members who are inactive with good reasons for over 2 weeks.
However, I would want to be able to keep some members, especially if we know that there are very special circumstances. For example, we have one member now who is on active duty in the military, and is in Iraq. I don't know what his particular circumstances are right now, but I wouldn't want to delete him anytime soon.
However, if someone has only posted their introduction, or hasn't posted at all, then I would say to delete them if they haven't visited the forum in a month. I understand how people might want to read, but not participate all that much, so would give them the opportunity to stay here if that is all they want to do. After all, maybe we are helping some of them.
However, I noticed that we have some members who posted a few times, possibly even reached mature status, then stopped visiting the forum altogether. That is sad. In those cases, if they haven't visited in 3 months, and haven't communicated with the Architect or the moderators, then I would remove them as well.