The average tenure of a CMO has shrunk even further, according to a new survey from Spencer Stuart, down from a sobering 23.6 months in 2004 and 23.5 months last year to an even tighter 23.2 months this year, AdAge reports (via MediaBuyerPlanner). There's no easy answer to why the window of opportunity slams shut so fast for the CMO. But marketing executives believe that while the pressure to perform is exacerbated by the short-term demands of Wall Street, they are also hindered by lack of access to CEOs.

A CMO Council study from last year found that top marketing executives admited that poor performance has led to a lack of influence within the corporate hierarchy. Only 10 percent of respondents to the CMO Council survey said their marketing groups are "highly influential and strategic" within the company. Less than half said their teams are "well regarded and respected," even though two-thirds of CEOs polled in a separate survey by Chief Executive magazine said their marketing groups are "mission critical" for creating top-line growth.

In Spencer Stuart's 2004 study, 41 percent of CMOs were appointed in the prior 12 months and only 14 percent of marketing heads had been with their firms for more than three years. Today, 53 percent of the executives have been in their jobs for less than a year. But the good news is that nearly a quarter have hung on for three or more, indicating that a corporate form of natural selection may be in play.

At least a dozen of the executives in the sample were general managers prior to their CMO posts and 20 percent had been a general manager at some point in their earlier careers.