I agree with aephi alum, though I'm not married - I feel as though I will just have to know.
But it differs for everyone. There are folks with 28 years of wedded bliss under their belts and they only knew each other for six months before MARRIAGE (not engagement, marriage!) and folks with 2 years of mediocracy and tolerance after knowing each other for 10 years.
ETA: Fleur de Lis, your comment reminded me of that saying, "Don't settle for the one you can live with, be with the one you can't live without." (Attribution?)