[a-priori]

A general rule is that you can live indefinitely if you withdraw 3-4% of your investments annually to pay living expenses. There are calculators out there (Google 'FIRECalc') that work out the odds of success (i.e, not going bankrupt) with various starting points and spending rates. At 3% there's about a 95% chance of success over a few decades, based on the past performance of the financial markets.

Let's say you spend $5k/month normally, as a round number. That works out to between $1.5M ($5000x12/3%) and $2M ($5000x12/4%) to sustain your living expenses indefinitely with no further income.