This is not a question with a precise answer (strictly speaking the answer would be best captured by a poll, but that functionality is not available), but I am genuinely interested in the answer, so I will ask it anyway.
Over the course of your career, how much time have you spent on greenfield development compared with brownfield?
Over the last 10 years I would estimate that I have spent 20% on greenfield and 80% on brownfield. Is this typical?
I think it's typical for professionals who deal with customers to spend more time in brownfield development. The reason is that customers typically aren't willing to throw out their existing software to adopt the "latest and greatest" (green) software.
Developers in research or academics, however, may be more likely to do greenfield development. Start-ups as well.
I think that your ratio 20:80 is representative of many/most developers. As to new development: if you are building software incrementally (Scrum, XP, etc) then one could argue that you spend almost all of your time in brownfield development. Except for the initial iteration/exploratory work, prototyping, even when you are building something new, you are already working on an established code base, refactoring and extending. So how much greenfield development is actually green?
Over the past decade or so, I've always worked on software that was used as the center of my company's business. (Both SaaS and a software product.) And while I've always come into the with an existing system (so brownfield), we've usually put out a ground-up redesign/rewrite (so greenfield.) So, to break to down:
So, that is seems to be the opposite of you. It is the nature of the companies I've sought out, and hence the projects. My software is our company's main product, and that means I work on the same code base for years, usually after having created it from scratch myself/ourselves.
And I like it that way.