As a quick note for Pokertracker users:
You can find your standard deviation by going to Session notes and clicking on "more detail"



Note in the example above that your average win for 100 hours is already greater than your standard deviation for 100 hours. When your average win becomes exactly equal to your standard deviation, you will be ahead more than 84% of the time.
This is because your results will lie within +/- 1 standard deviation from average 68% of the time, so 32% of the time they will lie outside this +/- interval. 16% of the time they will lie more than 1 standard deviation below the average, and 16% of the time they will lie more than 1 standard deviation above average.
This is based on a normal distribution or "Gaussian curve"

normal distribution: n. theoretical frequency distribution for a set of variable data, usually represented by a bell-shaped curve symmetrical about the mean. Also called Gaussian distribution.

Assuming $147 represents your true standard deviation for 1 hour, after 100 hours your average win will be $2500/$1470 = 1.7 standard deviations.
From a table of the standard normal distribution (I have posted one below), we can determine that you will be ahead more than 95% of the time at the end of this period. Note that with a different standard deviation, your probability of winning over X #of hands is much different.