After two weeks of working on a project you know whether it will work or not.
When you want a break from dogs and you take them to the kennel to the stars no one thinks you're a bad pet owner. But when you have kids you can't drop them off for three weeks without someone calling Child Protective Services!
It usually takes me more than three weeks to prepare a good impromptu speech.
The time to enjoy a European trip is about three weeks after unpacking.
I've been on a diet for two weeks and all I've lost is two weeks.
When you travel so many weeks a year it's always nice to have a home-cooked meal.