We were about 10-11 years in before marriage, figured with one kid and another one on the way it was time. I think it all depends on when you are ready and willing. Nothing should force you into it but a mutual decision between both of you.
I feel like its about the connection too, but I guess I'm kinda old fashioned, I think that if you're gonna be together, you should be married. I don't force this on other people, but if I've been in a relationship that long I want a big white dress and a ring on my finger, haha.
My husband proposed when we'd been together for just over a year (we'd known each other for 7 years at that point, having been classmates and friends since we were 12). We were engaged for 3 years, and finally tied the knot this past May (at which time, we'd been dating for 4.5 years).
It was never about how long we'd been together, but when it felt RIGHT. We knew early-on in our relationship that we never wanted to be apart, to be with anyone else ever again. When we realized that, we sat down and discussed a timeline for our life together. We wanted to marry at age 21 or 22 (check), then have kids around our mid-to-late twenties (26, preferably).
It's not about time but......
We got engaged when we were together 5 years and married at 6 years together. Since we started dating when I was 14 it wasn't likely that we would be getting married in the first 4 years.
It's not about a length of time. It depends on your current situation and how you feel in your relationship with your partner. My partner and I are getting married in December and we've been together 7 years this April.