I think the chances are extremely small because of the way in which our planet developed into a habitable planet.
First off you'd all the things discussed already. Such as liquid water, the right distance from the star, etc.
After all that you also have the probability of microscopic life developing and evolving to a point where all the life combined can change the conditions of the planet to be more friendly to higher forms of life. While at the same time avoiding catastrophic events that would bring the developmental level of lifeforms back down to the microscopic form.
So in order for a planet to be habitable for life (beyond the microscopic) it would need a significant amount of time to evolve to a level where it can transform the planet into a self adjusting biosphere that is required for advanced forms of life to be around.
The chances of there being a planet with the right conditions for microscopic life, then also the chances that that life can evolve with out being blown up back to the basic level... is extremely small
maybe even one per galaxy?
Then there is the matter of time. The chance of life existing at the same time that we are alive as a species is even smaller. There could be other planets developing life in our galaxy but the odds are that those planets have already developed the life and then died off, or have yet to develop.