A Dom should study, learn, and train his submissive until she is so happy to be used for his pleasure that the expression on her face is that of someone who is over-stimulated and too exhausted to do anything about it.

Regardless of which hole he is using, a good Dom will know his submissive so intimately that he is able to keep that expression on her face for however long he pleases.
Video post by CrownedKingOfControl