Expired Listings: How far back should you go?

[Ben Staples]

I'm an agent in the greater Boston area and am pulling expired listings to cold call / mail to.  For agents/wholesalers who have had success with expired listings, how far back should you go?  

For example: some people that I talk to say it is best to call as soon as they expire, others say I should pull a list that expired this time last year and check in.

Any thoughts, success stories OR failures would be welcome!