Ideally 25 months. But we can still work with any amount of sales history (even only one month) to make forecasts, although accuracy will increase with more history.
We need at least 12 months for seasonal trend analysis and application, but recommend 24 months.
We don't read any data older than 24 months since this can sometimes cause inaccuracies due to out of date trends. We focus on the most recent trends since these are the best predictors of upcoming demand.