I had an instructor that apparently pursued a career as CRI/IRI (on MEP/Instr.) after 10+ years. The reason, I believe his inability to find an airline job. To be fair, he was quite hard to work with. I never asked his salary, but it seemed survivable...
I'd say the time it takes to become CRI/IRI would depend on the flight school's need for such instructors. From my experience, 2-4 years instructing PPL & CPL before internal training to CRI/IRI.
Sry... all I can give you