What is the missing number?

Janice, my sister, had been pestering her husband, a carpenter, for more than a decade to build a screen door for the kitchen.
One day, to her delight, he built and installed one in less than two hours. It was both practical and pretty. She glanced towards the front door and wistfully remarked that one would look good there, as well.
"Are you kidding?" he gasped. "You can't just whip these things up, you know. It takes ten years to build a door like this."