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).