Do you remember when you first used this password at LinkedIn? It could help narrow the dates of the breach. Especially useful would be the presence of a strong password in the list that was subsequently changed. That might help determine its freshness, if the new password isn't present (although this may be an incomplete list from an ongoing breach).
I couldn't find my password on the list and I've been using the same password for LinkedIn since I registered. I was trying to remember when was that. If someone know how to find out the last time you changed your pass or when you registered for linkedIn please let me know. I'd guess I use linkedIn for over 4 years at least.
A "member since" date is available on the "Account & Settings" page. Choose "settings" in the drop down that appears when you hover over your (account) name in the upper right corner of any LinkedIn page.
I'm not a math person either, but here's some fodder for someone who is.
Mark Burnett's extensive password collection (which he acknowledges is skewed, because it's largely based on cracked passwords, he only harvests passwords between 3 and 30 chars, etc.). Here's how some of his stats shake out:
* Although my list contains about 6 million username/password combos, the list only contains about 1,300,000 unique passwords.
* Of those, approximately 300,000 of those passwords are used by more than one person; about 1,000,000 only appear once (and a good portion of those are obviously generated by a computer).
* The list of the top 20 passwords rarely changes and 1 out of every 50 people uses one of these passwords.
So it's conceivable that 6M unique passwords could cover a very significant portion of a 120M user namespace.