I've been playing my fortress for a couple of (in-game) years on low environmental threat, and so far haven't had any sightings of goblins at all.
Obviously this isn't very surprising, but I've now started a medium threat game, and I'm not sure when or what to expect danger-wise.
Is the threat I'm likely to face based solely on time (other than annoying caravans)? Or is it population? Is there a reasonably consistent pattern?