As a general rule, it's safe for many FIRE investors to retire when they have an extensive enough portfolio to sustain their spending habits, adjusted to inflation — which often comes out to about 4% of their portfolio's starting value. The rule isn't perfect and has been criticized for being too simplistic. Investment company, Vanguard, notes that it was designed for investors with a 30-year retirement window, whereas many FIRE devotees may be looking at 40 or 50 years.