We usually go through a pound of butter a week, between the things I cook/bake and the fact that there's almost always fresh bread on hand.
I keep one stick in the fridge, one stick in a butter dish on the counter, and the rest in the freezer.
The counter butter is eaten quickly in my house, usually 3-5 days. I've never had it go bad, and I have had some sit out for nearly two weeks. I do use salted butter, if that makes any difference.
Spreadable margarine is nothing compared to real butter, so I don't buy it, and spreadable real butter is obscenely expensive. I experimented with making my own spreadable butter, but I find that we're much happier just keeping a stick of butter on the counter.
We keep the house pretty cool, 65-68 in the winter and no more than 71-72 in the summer