The once-dominant team have not won a race since 2012 or stood on the podium since 2014.
The upheaval continued this week when racing director Eric Boullier, a Frenchman who joined from Lotus in 2014, resigned. Technical head Tim Goss was moved aside in April.
McLaren ended a three-year partnership with Honda last season but have also under-performed with the same Renault engines that have taken Red Bull to three victories in nine races.
"We've had a tough time for many years now, even really before Eric joined," Brown told reporters before the team's home British Grand Prix, describing the situation as uncompetitive and unacceptable.
It was going to take some time to contend for titles. "I think we are years away. I don’t know if that’s two or 10, or somewhere in-between," he said.
"Probably more like somewhere in-between, but I don’t want to get into predictions. I think we have to be very honest with ourselves and our fans that this is going to be a journey. I think everyone needs to recognise that."
McLaren are sixth in the constructors' standings and in danger of slipping to seventh, with Force India -- financially strapped and operating on a fraction of the budget -- only two points behind.
The team had said last year, when Honda took much of the flak, that they had one of the best chassis but Brown recognised on Thursday that had not been the case.
"No, definitely not," he said, adding that this year's was no better: "We know we have less downforce this year than last year."
McLaren driver Fernando Alonso told reporters separately that he trusted Brown 100 percent, although he had also had the same faith in Boullier.
The Spanish double world champion denied he was calling the shots at McLaren, with his former race engineer Andrea Stella promoted to performance director and Brazilian Gil de Ferran, an Indy 500 winner with whom Alonso worked at Indianapolis last year, brought in as sporting director.
"I just drive the cars," said Alonso. "With Zak obviously we have a very close relationship even if it's a short time that we met...He's a racer, he understands racing commitment and spirit and I think he's going to be a good leader for the future at McLaren."
(Reporting by Alan Baldwin, editing by David Stamp)
