Possible Clues:

• Santa's list-checking frequency
• Prudent way to think
• Double starts to take wonder drug
• Careful way to think
• How often, at most, one can be elected U.S. president
• How many times Bob Marley will "Do It"?
• How many times a day a stopped clock is right
• 'Measure ___ ...'
• You only live — (film)
• "Once bitten, ___ shy"