26 years for me, but I'd say it's only the past 16 years that I've actually been good at it. A matter of lacking experience, and slowly learning from DM's who were doing both the right things, and the wrong things. I only DM'd in the early days because there was no one else who would.
I've been playing, and DMing, on and off, for 22 years.
I have certain strengths when DMing, and some weaknesses. I try to emphasize my strengths to compensate for my weaknesses. I have never had any complaints, so I guess that means that I am pretty good.
However, no matter how good you are, you can always get better.
Me too. I was 9 years old when I discovered roleplaying games. The first games was with a friend, and each of us alternated between GM and player - each with 4 characters at the same time.
But I didn't count these years in the poll, as it wasn't "proper" GMing
I think I'm a good GM for the group I have. Not everybody would like my style, I'm sure, but we have a good time. I'm currently running my first-ever PbP, so you can ask those players yourself.
Started when I just turned 11. Ran every version of D&D except the original three books. During my tenure behind the screen I have run Shadowrun, Vampire, many different GURPS, In Nomine, Mage, Tunnels and Trolls, and several others I have probably forgotten. no one else in my varied and ever evolving group has any real desire to do so.
All started Labor Day Weekend, 1975. I owned the rules, so I had to be the GM. Since then, I've done about 75% GM work, 25% playing (and that might be a little too high on the playing side).
I consider myself pretty good; my players like me as a GM, so I think that counts a long way.