We do this almost every time we make love. I've even published an article on erotic massage. For us it's the perfect foreplay. I usually start with a back rub and move south pretty quickly. When she turns onto her back I settle in between her legs and stroke in various patterns from her breasts to her knees - gradually reducing the circle to build up her anticipation for the long awaited 'main event'.
The groin area is filled with nerve endings - and by now know exactly how she likes it - but also mix in some new moves to keep things fresh. Sometimes it progresses to oral sex - sometimes not - just depends. The slow build up gets us both ready both physically and mentally for the up-cumming orgasms.
Overall the process takes about 80% of love making time.