According to the PDN (Photo District News) survey,
PDN's Stock Income Survey: Earnings By Agency & Specialty
The average Microstock shooter made $4005 a year. That would be about 8000 downloads a year.
While there are some obvious flaws in a voluntary survey, made up of 1800 subscribers of a publication, it probably shows a higher interest and activity level than you would find with the average Microstock shooter. The people who took the survey, would be professionals and people making a serious effort, even if it was part time.
Average with Microstock is a nebulous measure. There are active people making over 100k a year, which means the average is skewed towards the high side. Then there are people who upload six photos and make nothing, which would drive the average of all microstock photographers, way down.
If DT, SS or IS, every released an "average" figure, that would be something to see. If it was by number of photographers, and showed the median income, it would be fantastic reading.
I don't see any way to make sense out of this, in any fair way, and come to some valid conclusion, of the potential to make a living at Microstock, other than, some people do very well and some people are spinning their wheels. It all depends.
It's the same as if someone came here and said, "what camera should I buy?" and expect some simple answer. There is none.
Myself, the question is, how good do I have to be, and how much time and effort does it take, to be "average"? :-D If it takes two years and $4000 in equipment, (that's everything, lenses, cameras, computer, software, lighting... everything) and so far I'm finding it takes at least 30-60 minutes to edit each photo, plus the time to set up and shoot. Is it worth investing in all of that, to eventually break even?
On the other hand, I already have pretty much "everything" so I'd be working to profit from what I already own.
The difference being, if someone goes out and buys all of this, to sell microstock, it's going to be an uphill battle. If one already owns equipment, it might be a potentially profit making situation.
Meanwhile, thanks to all the people here, and reading and studying, I know one thing. Diversity is the answer. I'll keep selling news photos, keep selling individual "art" photos and try to get better and add Stock to my income stream.
I'm also positive about the potential to make a living from selling Stock or Microstock.
It all depends!