All my adult life in the sense that I've had to write things for my jobs (these days I teach college biology, which means writing things for labs and lectures, but I used to do research, which meant writing papers and proposals). Fiction writing is something I've been doing off and on since I was a kid too, but there have been gaps (like when I was in grad school, I never finished any creative project I started save one short story that now languishes on the hard drive of a long-dead computer). I didn't finish my first novel draft or try submitting short stories to magazines until fairly recently, though. Likewise for studying the craft of writing fiction outside of a creative writing class or being involved in any writing communities.
So maybe I should say I've been writing fiction seriously for 2-3 years now.