For my wife, like couplers she could vary the amount of time. The usual minimum was around ten minutes of either oral sex or intercourse. Yes, she was able to orgasm when a guy went down on her, in fact she almost always had her first orgasm of the session that way. I've sometime seen her have as many as six to ten orgasms in one go round with one guy. She could easily bring herself off by using her fingers and certainly with a dildo.
But . . . . it didn't come naturally. She told me that when she was in college, even before she lost her virginity, she learned to masturbate. It took her a lot of time, a couple of years, until she got her masters in the subject. She said she practiced three or four times a week, and talked about it with a couple of her girlfriends and a professor's wife she made friends with - they gave her all kinds of invaluable tips. By the time I met her twelve years after that, she could make herself come by masturbation in just a couple of minutes.
She told me once that the reason she usually didn't come as quickly with a guy is that she could concentrate only on herself when masturbating. But with a guy - particularly a new guy - she had to find a rhythm and connection with him, as well as being distracted by her surroundings. She also told me in her fifties that the orgasms weren't the objective but shared pleasure with the guy (especially including me) was what she was aiming for.
The only times (very few of them) she didn't orgasm was when she was either feeling physical discomfort, or the guy wasn't willing to spend time pleasuring her.