I think that different publishers update their guidebooks at different intervals, and even for the same publisher, the updates may come at different intervals for different destinations, depending on need. I would look at the copyright page, where the dates (usually the years) of previous editions are listed. There you might see, for example, that the guidebook has been updated every 3 years. If that is the case with DK Eyewitness Toronto, then you might guess that the next edition may be coming out in 2016. But if it’s been updated every other year, and you’re not going until next year, you may want to wait for the new edition.