The general rule of thumb is roughly 24 hours from being taken orally, tho this can be influenced by matabolism and also mobility, if you lead a sedentary lifestyle it can take longer to "work its way thru". I have no idea which chemicals are involved in the variety of taste in semen, but some are absorbed and processed by the body quicker than others, such as alcohol. A large part of the fluid that is semen, contains fructose so a higher sugar intake results in a higher fructose level and therefore a sweeter taste "apparently". Fructose is the form of sugar most commonly found in fruit so this would make sense with fruit making a more pleasant tasting semen.
To go back to the origional point, my first few sexual encounters all swallowed so I assumed this was the norm. Later I found that it was roughly 50% swallowed and 50% didnt take into into the mouth at all, only 1 girl spat, which I found strange as if its in the mouth its already been tasted so why spit ? so it must be as much the consistency as the taste with it being likened to the thickness of mucus or liquids like Gaviscon (YUK)
Like a previous poster I always give a warning as, if I was someone who gave BJ's, I personally would never have it anywhere near my mouth and the mere thought of a snowball makes my stomach do somersaults.
I think that its wrong to expect someone to do something I am not prepared to do my self but being a typical male never complain if a girl likes to do it.
Also gotta agree with the "chilled wine" BJ and the warm tea BJ thats such an added bonus to the sensation