Standard Deviation questions.

[OrianasDaad]

I've done some searching, but came up with nothing very conclusive.

How many hands does a person have to play to have 90% confidence in their SD? 75%? How confident can a person be in their SD after say only 1000 hands?

My search results turned up some vague posts about how it needs to be "many, many hands". This, or course, is obvious, but I'm interested seeing how this is calculated.

My question is because it seems that people are always asking about winrate, and SD is often just a number that appears to be just "plugged-in", regardless of the confidence of that number.

Thanks in advance.