Bill Ford was booted after five years heading up Ford Motor Corporation (NYSE:F). Bristol Myers Squibb Co. (NYSE:BMY)ousted CEO Peter Dolan only a few days later; his tenure was also five years. Around the same time, it was announced that PepsiCo, Inc. (NYSE:PEP) CEO Steven Reinemund was stepping down to be replaced by up-and-comer Indra Nooyi. His time in the boardroom? Also five years. Jack Stahl was ousted as Revlon, Inc. (NYSE:REV) CEO after ... four-and-a-half years last Monday.

I think I see a pattern. If you're not cutting the cheese (and how) after the end of the fourth annual report under your tutelage, well then, it stands to reason you'll be shown the door. Or, ahem, suddenly find yourself needing to "spend more time with your family."

Michael Dell follows that pattern. Recently, he was faced with criticisms for his successor as CEO of Dell Inc. (NASDAQ:DELL), Kevin Rollins. Rollins' time at the helm? A bit more than two years. It's not enough, Dell said, and affirmed his support for his long-time right-hand man. Dell's troubles, he said, were not yet attributable to him.

Sumner Redstone, though, gave Viacom, Inc. (NYSE:VIA) CEO Tom Freston only eight months. In a move that affirmed Redstone's "eccentricity" (which in this context is a nice way of saying "impetuous" or "hot-tempered"), Mssr. Redstone blamed all his company's troubles on a man who'd only been CEO for less than a year ... hardly enough, you'd think, to evaluate his performance (or for his leadership to have any real impact on the company's stock price).

Is five years enough? Too much? Too little?