I think 90 days without new posts would be a good length for autolock just as general practice.

Sadly, it's not going to stop thread necromancy. The necromancers will simply hunt for topics about to get locked. Better for SE to establish more specific rules about bumping posts that have had no recent discussion then enforce that.

I wonder if there's a way to create a "softlock" on posts so posters could only reply to one post a day that has had no current discussion in more than 30 days. Someone with a legitimate need to revive an old topic could do so without trolls flooding the front page with old topics.