I'm in the USA. One agency portfolio is paying about 20 cents Per Image Per Year. Another Agency portfolio about 60 cents PIPY. Image exclusive in both agencies.
I started stock in 2002 as a hobby and kept most years net cash positive. About 13 years ago, my high months supported about half of two kids in private college and all photography expenses including some trips. Today I can't cover mileage, camera repair, Internet bill, and tax accountant. My good stock run is pretty much over and is back to being a subsidized hobby.
"Hitting the wall" is very real. That is, reaching a peak of sales and not being able to continue the climb. The reasons and nuances that culminate to hit the wall are from many attack vectors and are varied per each photog. Subject matter, industry styles, competition, agency pay and business structures, personal motivation, photo business cost controls, and many other variables, contribute to the wall.