How long does it take a woman or climax from oral once she's been brought as far as foreplay can take her?

I don't have this experience with that many women, but I had this one girlfriend who never took as long as 30 seconds and I must have gone down on her several dozen times, but after a good dose of foreplay.
Now that I'm researching the issue, I'm finding numbers like 15 or 20 minutes, with 30 seconds seemingly like 1 in a million. But I'm wondering when they start the clock. When he goes straight down right away or after they've spent 15 minutes warming her up?