Assuming that "this generation" is meant to be marked from the time Israel was founded again as an independant nation or from the retaking of Jerusalem (and that's a major assumption), God could easily wait the better part of a century from that point (another 40-60 years from the present day) before the generation that witnessed the event "passed away."
Of course, it could also be that Yeshua was speaking of the generation that saw "all these things" described through the chapter, or that He was using genea (or whatever Hebrew word Mattityahu [Matthew] was translating) to refer to the Jewish people as a whole. In either case, trying to calculate the time of the end from this statement would be incorrect.
It seems clear to me from the lucan context that he was speaking of the generation that witnessed those things he was referring to at that point in His narration.