To make a living with photography (outside of commercial photograpy), one needs to deal in a variety of activities. Very, very few photographers do wildlife or landscapes, or any fine art photography, and make a living solely from picture sales. Most give seminars, conduct tours, write for magazines, teach photography, in additon to selling prints. Wildlife photography is very competitive, in addition to being very expensive, unless someone pays your way, and different and unusual shots usually take considerable time and resources to come by.
My suggestion is---unless you've inherited an extensive portfolio--don't quit your day job. If you don't have one, get one. It will at least subsidize your pixel/film vices, and, if you're successful with a photobusiness, it would give you more confidence in determining when it's time to jump into the deep end.
Again, this is my experience, but a lot depends on what you want as a life style and what you might be willing to give up to make a go at photography.
There is also the business side of things, and for me, this has always been the hardest. It's one thing to take pictures, good pictures, but its another thing to market them. I probably spend about 75% of my time on the marketing side--not that that's what I like to do, but that's what I have to do. Someone who's lucky enough to have an agent might be able to flip that number.
In any case, for someone that is just merging into the business, proceed with caution, get plenty of advise, and above all, keep it fun.