Statistically Significant

I'm sure it's been asked a million times before, but I keep hearing varying answers. I just posed this question to the group, but we got sidetracked on another topic. When computing blackjack strategy, be it card counting or BS, how many hands need to be dealt before you finally reach the statistically significant threshold, where it's no longer short term but now long term? Is it 10 million, 100 million, more? Just wondering for my own computations.

Thanks!