If I'm doing my math correctly, you need to charge $16,667 to earn a $500 certificate for non-NCL related purchases. If you were to have only NCL charges, the charges required to earn $500 would be $12,500.
I earned my first $500 certificate in about 6 months -- it's amazing how fast it builds up if you consciously try to charge on the card. Our average bill is somewhere between $1500 and $2000 -- but has been substantially higher the months when the kids tuition and books are due. It really does turn out to be one of the better "travel rewards" cards available.