Escalating danger (fun) over time
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?
What version are you playing? If you're in 31.14, the goblins will never invade due to a bug. Alternatively, it's possible that the area you selected had no local goblins -- when you were originally founding the fort, did the civilization screen list goblins? (keep TABing until you find it)
The two principal factors in determining ambushes, sieges, etc., are Fortress Wealth and Population. As both of those go up, ambushes occur more and more often, and eventually you'll cross the threshold (100,000 wealth and 80 people, I believe) that makes you eligible for sieges. Lucky you.