I would think that 100% of married Christians who truly adhere to Biblical teachings attend "house-church" every day. After all, in a marriage there are always 2 people. In a Christian marriage they are both believers. And finally, if they are following His teachings they are gathered "in His name." At least that's how it should be. It's what I work towards. Repeat after me: A church is not a building.
Pope John Paul II emphasized this idea of the family as church. The "domestic church". He once exhorted all families: "Families, become what you are!" - explaining that the family is the ideally-designed (By God, of course) "community of persons" that can remind us - right where we grow up - of the perfect community of persons, the Holy Trinity. That is what we try to do in my family.