[pzhon]

There shouldn't be a set number of hands at which point your win rate is considered sustained. What you can say is that your confidence interval tightens about your observed win rate.

A rough 95% confidence interval is your observed win rate +- 2 standard deviations. The standard deviation of your win rate after n * 100 hands is SD/squareroot(n), where SD is your standard deviation per 100 hands. For example, if your standard deviation in limit is a typical 15 BB/100, and you played 5000 hands, n=50, and the standard deviation of your win rate is about 15/sqrt(50) ~ 2 BB/100. If your observed win rate is 1 BB/100, then your 95% confidence interval would be 1+-4 BB/100, from -3 BB/100 to +5 BB/100. Quadrupling the number of hands shrinks the confidence interval by a factor of 2.

In NL, the standard deviation depends more on your playing style. Typical values might be 85 big blinds/100 in full ring, and 95 big blinds/100 shorthanded, but some styles will produce much lower or much higher standard deviations.