I am not that sensitive after cumming and really enjoy post-orgasm oral. 
Unfortunately, I don't get enough opportunities to experience it 
 
I am very oral-centric and my guess is that guys who are oral-centric will enjoy it. Or maybe its the other way around and guys who are less sensitive are naturally oral-centric because it is more pleasurable than uncomfortable for them.
Have no idea of time because all I was thinking was hope this never ends 

 I guess around 3-5 mins.
p.s.: Makes a big difference if hot tea or water is used during the oral. 
Never in my life did I know that tea can be so addictive!!