Following the rule of thumb that one can spend about 4% of investment a year for it to remain sustainable, it's sufficient to accumulate about 25 times more than you spend in a year, which at 80% saving rate can be achieved in 6 years (more to reduce risk and/or accommodate possible future increase in spending (above inflation)).
There happens to be an article in the New York Times today about the 4% rule, based on a new paper titled The 4 Percent Rule is Not Safe in a Low-Yield World. It also seems worth noting that the 4% rule assumes a payout period of 30 years, so it's not entirely applicable for the purposes of this thread.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.