I fit into the category of hobbyist earning a buck on the side from photography - my main job is in the financial/insurance industry for the past 16 years. Other than having an online gallery up, and prints on the walls of my office, I don't actively market my photography. I pick up some sales at the office from people browsing - often clients or service personnel visiting the office for other reasons. I've picked up a few stock sales and a few publisher sales from the website, unsolicited. And I've done 5 or 6 hired shoots, also unsolicited, from people who admired my work and asked if I do shoots. My total income from photography? Probably in the range of $6,000. Over 3 years since my first sales...that makes it...$2,000 per year! Not much of a living, that's for sure. But I don't actively market as a photographer, and am quite happy if my hobby can essentially pay for itself. It has bought my last two cameras and 2 lenses, bag, and tripod...so no complaints so far.
I have a friend who has been a lifelong photographer/videographer - mostly wedding/event photography, but he has had to supplement his income with video, television, and commercial video work at times. In a good year, he brings in the vicinity of $45,000 - working all weekends and one day a week shooting, and in the vicinity of 4-6 hours a day editing, post-processing, cropping, printing, burning CDs, duplicating, etc. In a bad year (economy crunch has hurt his business lately as many consider photography a cuttable expense) he might struggle to bring in $20,000. Consider that he must commute a lot, pay for very expensive video and photography gear (including lighting set ups, filter sets, professional quality video cameras, editing software and memory/video intensive computer systems, and maintenance & supplies...he is close to breaking even in a bad year. He's done this for 20 years now.
Of course, there are others in the business who earn many times more. Wedding/event photographers who are very successful usually tend to earn more when they branch off into owning a company or studio and hiring others to do the photography for them - I don't know how much you can consider that 'earning a living from photography' at least from the standpoint of being the photographer anymore. But it is certainly something that you built up to from photography. Others who get into the hired-gun world, especially working for a publication as a staff photographer, can earn a much more comfortable living. And that doesn't even include the lucrative world of photography talks, walks, websites, lessons, books, etc that can come with being a published and established photographer.
Sadly, it's true - professional photographers can earn anywhere from $100 a year to millions. There's really no way to tell someone a precise number to expect from photography. I know I enjoy making some money from photography on the side, and I'd love if it was 5-figures or more a year - but at the same time I am very happy I have another, stable career to earn a living from.