Generally, if you have a lot of cash to start with, this is not a terrible idea. It doesn't need to be property though. Investing in the stock market is almost as profitable (10% avg annual return, vs 11%) but less volatile (
http://www.investopedia.com/ask/answers/052015/which-has-per...). Stocks also have a drastically lower barrier to entry, and are far quicker to buy/sell.
There's a reason that investment funds and big banking aren't primarily buying up all the housing in the world - it's typically not the most efficient return on investment.
The maths is fun though. How much do you need to make a year? Let's say $60k (approx US household average). Times by 10 (assuming the average 10% annual return) to get the investments you need for that return (so $600,000). If you can get that much capital, you can make that salary just from investment alone (bearing in mind that this is an average: it's not reliable, but if you've got a buffer you're _probably_ ok). In the long-term you'll want a bit more to keep up with inflation, but it's a reasonable ballpark. This is obviously passive income, so you've then got the rest of your time to earn extra in whatever form you'd like, with a lot of freedom and no pressure. The hard part is having the upfront cash...
(Obviously this isn't real financial advice. Don't make life decisions based on HN comments)