Since 2004 - 19 years! I moved to it because at the time I kept getting stuck in dependency hell on other distributions I'd tried and Portage at least gave me the knowledge and tools to fix things myself.
Since 2013 I think 10 years with the same two installs.
Whenever arch switched to pulse audio because Ubuntu did it.
But it was completely broken on my setup so the question that drove me to gentoo was
“How do I rollback?” the answer was use the previous packages in the cache which I had cleaned :/.
Using portage is so much more powerful and does exactly what it says, and there’s is usually always a way to get back to a working state.
17 years here. :) I did a bit of distros hopping (surprisingly, the one I loved the most beside Gentoo was KDE Neon, for how polished the interface experience was), but always had a least one gentoo running home. Now all my machines are Gentoo again, it's just the most stable distros for me (if anything, because it doesn't try to be smart and let me configure things my way).
Also, now that I'm confident patching big C programs, this is the dream distro for tinkering. I just fork ebuilds in my own portage tree, add my patches in the ebuilds, and just like that, I have patched programs managed by my package manager. Still need to do it with each program update, though, so it's cool, but not perfect.
Gentoo for 7 Years on and off but I've been using the same install for 5 years now. I wanted to challenge myself and use it as my main OS but I soon found out that it wasn't as hard as people made it out to be and it was actually fun setting up my system from scratch. With the existence of overlays, use-flags and lots of community support there's no reason for me to use another distro, Although I still get the itch to try out openSUSE, Mageia, and FreeBSD once in a while.