PG&E Corp.’s chief executive said Friday that it could take as long as 10 years for the company to improve its electric system enough to significantly diminish the need to pull the plug on customers to reduce the risk of sparking fires.

Bill Johnson, who joined the company in May, made the disclosure at a California Public Utilities Commission hearing where the panel’s president, Marybel Batjer, sharply criticized the company’s “inadequate execution” of a shut-off in which it turned off power to large portions of Northern...