How much data is good for forecasting?

Forecasting can be done with any length of data. However, what changes is the use of statistical method and consequently the accuracy of the outcome. The shorter history would mean using simple methods like moving averages while longer history would mean more powerful methods like exponential smoothing or ARIMA. A good forecasting software like Planamind or Forecast Pro will automatically pick up the best method basis the historical length and the inherent demand pattern.