By Rachel Feintzeig 

When John Chambers steps down from the top job at Cisco Systems Inc. this summer, it will end the run of one of the longest-serving corporate chieftains in the S&P 500.

The 65-year-old Mr. Chambers ran the tech giant for 20 years, during which it was briefly the world's most valuable company. This week, the company said that Mr. Chambers will pass the baton to Chuck Robbins, a 17-year Cisco veteran, following years of speculation about the company's succession plans.

Several long-tenured chief executives have left their posts recently, including Oracle Corp.'s Larry Ellison, now chief technology officer and chairman of the firm he led for 37 years, and Peter Rose, who spent 26 years leading Expeditors International of Washington.

Yet there are still plenty of corporate chieftains with decades under their belts, and those men show few signs of handing over power any time soon, said Peter Crist, the chairman of Chicago-based executive search firm Crist|Kolder Associates.

According to a list compiled by the Conference Board, the longest-tenured CEO in the S&P 500 is Leslie Wexner, who has served for more than a half-century atop L Brands Inc., which owns Victoria's Secret and other retailers.

Next is Berkshire Hathaway Inc.'s Warren Buffett, CEO for 45 years, and Alan Miller, who has spent 37 years at the helm of Universal Health Services Inc., an operator of hospitals and clinics.

Rupert Murdoch has spent 36 years atop 21st Century Fox Inc.; he is also the chairman of News Corp, the parent company of The Wall Street Journal.

It is unlikely, however, that new leaders like Mr. Robbins will enjoy the long runs of their predecessors, said Mr. Crist, since the top job today comes with more pressure and a heightened focus on the short-term.

Average CEO tenure might drop as baby-boomer chiefs retire, said Matteo Tonello, who oversees research about CEO succession for the Conference Board.

Tenure began to edge downward at the beginning of the last decade, according to Mr. Tonello, amid tightening corporate governance practices, stronger independent boards and CEOs' pay becoming tied more closely to performance. Tenure for S&P 500 chiefs was 9.9 years last year, up from 7.2 years during the peak of the financial crisis in 2009.

Ideal CEO tenure might be even shorter. Researchers at Sichuan University in Chengdu, China, and at the University of Texas at Arlington found that bosses lose touch with market conditions and become entrenched the longer they remain in their roles. The optimal tenure length, the researchers decided, was 4.8 years.

Most of today's long-standing executives have one crucial advantage: they are either founders or the descendants of founders, which gives them more control than the average public-company chief. Jeff Bezos, the founder and CEO of Amazon Inc., who has been at the helm since 1994, and Facebook Inc. co-founder Mark Zuckerberg are both current chiefs with large stakes in their firms.

Such leaders are tougher targets for activists, said Mr. Crist. He said he doesn't expect any members of the list will be forced out before they retire and expects some may hang on for years to come.

"Lots of CEOs basically live through their companies," he said. "It's their identities. So they aren't going to leave any time soon."

That doesn't stop anyone from speculating about the succession plans of leaders like Mr. Buffett. The Berkshire Hathaway chief turns 85 in August and has in recent years tried to assure shareholders that the company will be just fine when he's no longer at the helm.

After Mr. Buffett and his contemporaries move on, Mr. Crist said their replacements will have a tougher time weathering the pressures from activist investors and empowered boards.

"I think when the top 10, 15 names fade away you won't see in our lifetime anybody who's going to be a CEO for 60 or 50 years," Mr. Crist said. "It just will not happen, period."

Write to Rachel Feintzeig at rachel.feintzeig@wsj.com

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