The longest one that comes to mind for me was one day when my wife and I were commuting to downtown DC, we went through Old Town Alexandria using the right-lane HOV-2 lane on Route 1 and we somehow got every light as a green light all the way from the Beltway to the north end of Crystal City where the road becomes controlled-access. That's never happened to me before or since. I don't remember when it was, but I'm sure it must have been the day before a holiday weekend with lighter commute traffic. It wouldn't have been the week between Christmas and New Year's because the traffic is so light that week that we took I-395 instead. I've made it all the way through Old Town Alexandria on the green wave more than once, but never all the way up through Potomac Yard and Crystal City as well. (Even hitting the green wave can be difficult because you need to hit it
just right. If you're second on line at a red light approaching the twinned one-way streets in Old Town, odds are you won't catch the green wave because the guy in front of you will be too slow off the line, and you need to pace your speed perfectly because if you go the speed limit, you don't catch the green wave, but if you go 5 mph over, you'll be going just a little too fast.)
For those unfamiliar, here is a map. It's only four and a half miles, but during the morning rush hour it can easily take half an hour or more due to a couple of long red lights.
I don't remember how long it took to make that drive the day we hit all the lights, but I believe we were at Memorial Bridge less than 20 minutes after exiting the Beltway.