My girl does this daily. We call it "suckling", for when she simply wants my flacid cock i'm her mouth versus her actually sucking, which of course denotes a blowjob. That way I can ask for either and she knows exactly what I'm wanting.
It's a very intimate and wonderful experience. It feels amazing of course. I love the fact that there's absolutely no expectation of rushing, getting hard or cumming. If I do, fine. She'll drink me and continue to suckle, sometimes well over an hour or more, til she feels satisfied.
When bringing me my coffee first thing in the morning, she'll suckle me in bed as I wake up, sipping on my coffee. it always starts my day off in a great mood.
When at home, she'll always find the opportunity to suckle when we're just lounging or not having sex, like while watching a movie or show together, when i'm playing a video game on occasion, but my favorite is at night, when we go to bed, she'll slide down in kind of an L shape, get comfortable, lay her head on the inner part of my right leg, my left leg draped over her waist, flacid cock in her mouth, suckling, as we drift off to sleep.
and Let me say, it's innate, just like a baby, she'll stir in her sleep and begin to suckle again often throughout the night, waking me briefly with that pleasant suckling sensation, until moments later we both drift off back to sleep. If I cum, she'll drink all of me, half asleep half awake, before drifting off back to sleep.