When does BB/hr become a good measure?

[hackers238]

Due to the huge swings that I always see in NL, when does the BB/hr measure become a good measure of how well you're doing? 1000 hands? 10000 hands? 100000 hands?

I find that there is so much variance that it's hard to use this as a measure. 1 hand can make the difference between 3 BB/hr and -3 BB/hr.