As I said, he COULD reach Hewitt's level.
By sealing the YE no 1, Nadal is GUARANTEED at least another 15 weeks as no. 1, taking him to 76 weeks total. I'm not even sure whether it is mathematically possible for anyone to overtake him within the first 5 weeks of next year (highly unlikely as Nadal will still have 3 slams + at least 3 AMS to his name).
So I'll put my neck out and say Nadal will have beaten Hewitt's weeks at no. 1 before he next falls down the rankings.