i was assuming 20 trips, actually, but this must vary a huge amount depending on the kind of flying people do (commuting consultant vs long-haul int'l fliers, etc.) and these days, how much you're using credit card spend to get diamond vs. flying affects this. without doing a full analysis i'd guess i was usually doing 25 trips/year for diamond over the years, but i'd often do a lot of long haul, expensive flights for my job.