In general I treat any relationship lasting more than about five years without a major destabilizing crisis as stable. The relationships I'm thinking about... hm. I'd estimate the mean to be about seven years. Of course, the mean relationship time of my cohort increases as I get older.

It's also worth asking how one should treat open relationships. That is, I know a lot of stable couples who have had third and fourth parties involved in their relationship for a year or two or four; I don't consider those stable triads or quads, but neither do I consider them monogamous relationships.

I should probably also note that I reject the pervasive cultural notion that a relationship that ends in something other than death has somehow failed.