How should I set the sales history period value?

This value is under the demand analysis screen, below the sales and forecasts graph. It allows you to customize how much sales history the system uses when calculating the forecasts for an item. 

Generally speaking, you should not need to configure this, and the system will calculate the forecasts for you automatically based on your overall trend. 

However, if you know your more recent sales are going to be more indicative of your trend into the future you can shorten this so that your earlier sales aren't taken into account. 

For example, if you have had an uptick in sales over the last 3 months and this trend will continue, set the sales history period to be 3 months so that the system uses these sales to predict your forecasts into the future.