That depends on how old they are when they retire and if they have other income.
I know a couple who both retired recently. They had saved just short of $1 million in various retirement plans. Using the 4% rule - which says that you should be able to average a 4% return on your investments, some years better, some years worse - you will always have at least what you started with while taking that 4% in income every year.
That would be $40k they could take and keep their $1 million working for them. Add that $40k to their employer pension (paid by the company), retirement annuity (paid by employer and employee contributions) and Social Security, their income is slightly above what they were earning at the time they retired. Their pension/annuity and SS have cost-of-living provisions but their take from their savings will not grow unless their savings grow beyond the $1 million, so inflation can reduce their spending power over time. Which is why the age you retire is key. This couple, barring anything catastrophic, will not outlive their money. $1 million in savings along with their other income should be more than enough. For someone that wants to retire at 50, $1 million would never be enough and even $2 million might not be enough, at the 4% draw.