About once a week it seems, we are reminded that CMO tenure is unusually short. I have heard figures ranging from 23 months to 45 months. Both of these numbers are astonishingly low compared to other execs in the C-Suite: eight years for CEOs and ten years for CFOs. So why is CMO tenure so short? Experts have pointed to a host of reasons: the explosion of social media, the rise of big data, general complexity and chaos, incompetence…