The short answer to this question is two years' worth of data.

The long answer, however, is that while the two years' worth of data goalpost is ideal for us to generate an accurate forecast, our tool will also run with a smaller dataset. We will extrapolate the data you do have and use it to generate the forecast and fill in any gaps with the best match we can find, though the end result will not be as accurate. The larger the data pool, the more accurate the forecast is.