The back story was that I was writing some R code and I got to a point where I was stuck: there was something I wanted to do and I remembered that there was a built-in function that could accomplish my goal. Unfortunately, I couldn't remember that function's name. After some fruitless googling, I posted the question on SO.
So, how long did it actually take to get the right answer? About 6 1/2 minutes. As I write this sentence, I'm waiting for some more time to elapse so I can actually approve the answer:
This has been my general experience with SO---amazingly high-quality answers delivered almost immediately. I feel sheepish that I haven't been able to answer as many questions as I've asked, but one of the animating ideas of the community is that asking high-quality, answerable questions is a way of contributing.
What's interesting to me is that SO is an example of a primarily social---as opposed to technological---innovation. There's nothing really technically innovative about SO: the site is fast, search works well, tagging works well etc., but lots of sites have those things. What's special about SO is that through a carefully designed system of incentives and policies, they have created a community that is literally---and I think profoundly---changing how people program computers.
The reason I point about the social nature of the innovation is that it's become popular to lament the shallowness or perceived frivolity of many start-ups that are built around social rather than technological innovations (e.g., Facebook, Twitter, Instagram etc.). The idea seems to be that if you aren't making solar panels or cancer-curing drugs, you're not doing something socially useful. I personally don't share that bias, but if we are going to judge companies on the basis of some more "serious" metric like productivity or social surplus, then SO is a great example how a purely social innovation can succeed spectacularly on those metrics.