As long as it takes! I like making sure my partner is having a good time. I feel it helps most when you're not in a rush and/or making your partner uncomfortable about not coming "quickly enough" or otherwise giving them orgasm anxiety. If you're going down on someone but think that there's a time frame, you're doing it for the wrong reasons.
As long as it takes. I have an oral fixation and really enjoy it. I pace myself, using various techniques to get him "there". Sometimes I edge him and keep him holding back for awhile because it's fun for both of us. I suppose the general amount of time it takes with making that the fastest goal possible is around 10 minutes. But we choose to take longer sometimes.
My wife and I only do oral when we have enough time not to have to worry about how long it takes. I don't think I ever paid attention to how long she takes. I love giving her head, so the longer the better. I like to give her as many little and big "earthquakes" as she wants. Usually I keep going for one more "O" after she asks me to stop!
I used to give head to a guy, many years ago. I do know I gave him head for almost 2 hours once. He was watching porn with me on my knees at his feet. After he came the first time, he told me to keep his penis in my mouth until it got hard again and then suck him off again--which I LOVED!
I don't shoot (so to speak) for a specific time. Sometimes oral is a great way to get off fast, and other times I like nothing more than to "ride the edge" while I am being orally stimulated...for as long as possible.