The longest I've ever worked at a single company was four and a half years, from August of 1996, when I joined what was then Maximizer Technologies, through January 2001, when the company (by then called Multiactive Software) let me go in the second of several rounds of layoffs. I came to Maximizer from a short and unpleasant stint in the advertising department of Gardens West magazine, after two years as a full-time musician.
While I'd been geeky since the 1970s, my mad computer skillz had declined a bit during my tenure in the band (even though we were apparently the first indie group to run our own email list), and, having been an Apple guy since 1982 or so, I'd hardly had any exposure to Windows at all. Gardens West showed me Windows 95, but Maximizer—where I started as an administrative assistant in the Development department, but moved through technical writing, running the website, and creating the whole company's style guide—got me into its depths, where I helped craft installers, create tutorials and demonstration data, build C++ language coding guidelines, and delve deep into the horrors of the Windows Registry.
Now, I'm still a Mac guy, and while I kept a Windows machine at home for several years for testing and doing necessary work stuff, when it died I didn't bother to replace it. But Maximizer/Multiactive was what turned me into a real geek, with the skills to troubleshoot friends' computers regardless of operating system (although my very limited Linux abilities owe more to early Internet experience on UBC's "unixg" network). In early 1997, Maximizer was where I learned to write HTML, and within months I was managing the company website, as well as creating this one—which still contains archaeological bits and pieces of those first halting attempts at web code. (One example is the odd dimensions of my portrait that appears on most pages.) It was also where I was working when both my children were born. Since parenthood is what really forces you to grow up, in the most profound respects, I started there still something of a boy, and left as a man.
As of this summer, only now have I been away from Maximizer longer than I worked there. I've had my fabulous "new" job at Navarik for close to two years, and expect to stay much longer than that. But the stint at Maximizer—probably at least 5% of my life, it turns out—was what let me spend the year and a half before Navarik making good money as a freelance technical writer. I was happy to leave, and my current job is much better, but I'm also glad to have been there.