It's a function of percentage income saved, not amount earned. The formula is (income / investment rate / percent income saved). If my investment rate is 4%, I need to save 25 years worth of salary. Saving 50% of my salary looks like I would need 50 years on the surface. However, my portfolio generates income from day 1, so it's actually a lot faster than it seems.

Just for reference, I was living on less than $750/month until I got married. The companies I worked at had plenty of free food. I didn't need a gym membership because the handyman work I did on my investment properties was more than enough. Until we had kids, my spouse and I were still able to survive on less than $1,000/month. We are based in the south SF Bay Area. We had 100% income replacement by our mid-30s.