I don't know about you but most of the women I have been with give up well before the 20 minute mark.
So, if time is a factor, as I'd probably agree, should we try to find out at which point it is no longer ok not to return the favor? For instance, if she has been pleasuring you for 2 minutes, it would be acceptable to go straight to sex, but if she has spent anything from 3 to 30 minutes (at 30+, I guess sex is out of the question anyway), you are obliged to spend at least the same amount of time on her.
To be honest, I don't think using reason is appropriate in cases of intimacy/sex/love. Instead, one should rely on what "feels" right. To me it doesn't feel right to not return something which has been given to me, it being a blow job or some other act of kindness. I don't act according to kantian principles, for instance, then, (even though I might act in accordance with it) but follow my own sense of what is the "right thing to do", which is mostly, in this particular case, based on emotion. If returning acts of kindness is something that doesn't fall naturally to someone, that someone is a douche. I call it the "being a douche" principle.
With that said, doing an act of kindness only because one feels obliged to do it is arguably not what most would consider virtuous, but then again, some think that that's more virtuous than someone doing an act of kindness without it having any "cost" to the one doing it. They haven't sacrificed anything to do that kindness (i.e, he was horny, and couldn't imagine something more pleasurable than going down on the woman), while you, when you go down on someone that smells without wanting to, you act against your will because you figure it to be the just and moral thing to do - now, isn't that a nice thing to do? :P
After all, in the first case, you just have some horny dude acting on impulses, while in the latter case, you have someone that does something to pleasure someone else - and only to pleasure someone else - as he, personally, would rather stick his tongue in a thrash can.