It is worth noting first that Real Estate is by no means passive income. The amount of effort and cost involved (maintenance, legal, advertising, insurance, finding the properties, ect.) can be staggering and require a good amount of specialized knowledge to do well. The amount you would have to pay a management company to do the work for you especially with only a few properties can wipe out much of the income while you keep the risk.
However, keshlam's answer still applies pretty well in this case but with a lot more variability. One million dollars worth of property should get you there on average less if you do much of the work yourself. However, real estate because it is so local and done in ~100k chunks is a lot more variable than passive stocks and bonds, for instance, as you can get really lucky or really unlucky with location, the local economy, natural disasters, tenants...
Taking out loans to get you to the million worth of property faster but can add a lot more risk to the process. Including the risk you wouldn't have any money on retirement. Investing in Real Estate can be a faster way to retirement than some, but it is more risky than many and definitely not passive.