When is the best time to buy the newest edition of a guide book?

I often buy a guide book before I go on a trip. I usually get DK Eyewitness (Top 10) or Lonely Planet. I’m wondering how often they publish a new edition (yearly, every two years, or more?). Right now I’m looking to get the DK Top 10 Toronto published on April 15, 2013. Do you think a new edition is coming out soon or is it most likely they won’t have a new one again until next year? How often do they update these guide books?