By definition, butternuts are a storage crop meaning that if you store them properly, you can keep them for months! They like a cool, dry place with good airflow and lack of sunlight. Check it regularly for signs of softness, mold or wrinkles; if you notice those developing early, you can easily save most of it by preparing the squash promptly and cutting out the soft parts.
The way we prefer to prepare butternut squash:
New Whole Foods Encyclopedia by Rebecca Wood
Produce: A Fruit and Vegetable Lover's Guide
And, our own experience!