In my experience, a lot depends on the woman. I love to eat pussy, love seeing, feeling, hearing her cum. Some of my partners were difficult to get off...lots of time and effort required; others, not so much. It helps to know the woman well...the right spot, pressure, cadence...etc...the signs that she is close to orgasm. My wife was always an easy orgasm...not that I am that skilled. After many years together I can get her off in 2 - 3 minutes or keep her on the edge for 10 - 15 minutes - until she begs me to finish her.
Making a woman you love cum is one of the great pleasures of being a man. Given the choice of going down on her or her going down on me, I would go down on her.