Check out Mr Money Moustache, he explains the (rather simple) math behind "retiring early"
https://www.mrmoneymustache.com/2012/01/13/the-shockingly-simple-math-behind-early-retirement/

http://www.mrmoneymustache.com/2011/09/15/a-brief-history-of-the-stash-how-we-saved-from-zero-to-retirement-in-ten-years/