[gjm11]

That depends entirely on what sort of lifestyle you're trying to sustain.

There shouldn't be much difference between the level of savings needed to retire at (say) 35 and (say) 45. In either case, you want enough that with low-end-of-plausible stock market growth, you're taking out less than growth is putting back.

There's some research around what level of growth it's safe to assume; how much you trust it depends on whether you think the past behaviour of the US stock market is a good predictor of the future behaviour of whatever you invest in. I suggest (but this is Not Financial Advice and I am Not a Financial Advisor) working on the basis that you can probably get ~2.5% of your investment out per year pretty safely. (Usually it'll grow more than that, even after inflation. That cushions you a bit against cases where bad things happen. I assume you are willing to trim your lifestyle back a bit in bad times.)

That means you need about 40 years' worth of spending in your savings. That would be $5M if you are spending $125k/year. In my opinion (about which you need not care), if you are spending $125k/year then either you are earning so much that saying $5M isn't going to be a big challenge for you, or else something is amiss with your priorities.