Really, we don't, and the hype surrounding the latest Kepler-based estimate is monumentally annoying. First, the "habitable zone" they use spans from Venus to Mars, and as you can tell from our own system, just being within that zone doesn't make a planet "earth-like". Second, their estimate includes planets up to twice Earth's diameter, which could mean six times its mass, which doesn't sound very "earth-like" to me. Third, the data is all very preliminary and requires corrections like this one:
Similarly, the team accounted for missed planets in their analysis, as well as the fact that only a small fraction of planets follow orbits that allow their crossings in front of their host star to be seen from Earth.So whatever the actual percentage of sun-like stars that have earth-like planets turns out to be, I would bet my house it isn't the 22% these guys came up with.
We have learned over the past 20 years that planets are common and diverse, including types we did not know were possible. But that's about all we know right now.