Can you walk through the math of how many years it takes to reach FI?

The estimate combines three inputs: your savings rate, your real investment return, and the target of 25x expenses. Because you save part of your income and invest it, the calculation is a future-value-of-an-annuity problem, not simple division.

Assuming a 5% real return and stopping at 25x spending, the widely cited results are roughly: 10% savings rate takes about 51 years, 20% about 37 years, 30% about 28 years, 40% about 22 years, 50% about 17 years, and 60% about 12.5 years. Starting savings shorten this further. The headline lesson is that the curve is steep at low rates and flattens as you climb, so early increases in your savings rate pay off enormously. Run your own numbers at wealthserene.com/tools/fire-calculator.