Question: how many data points for time series?
Answer: Hanke and Wichern, chapter 3, page 80 ( http://www.amazon.com/Business-Forecasting-Edition-John-Hanke/dp/0132301202 ) recommend a minimum 2xs to 6xs depending on the method (where s is the seasonal period, so s=12 for monthly data). 50 data points would be 50/12 = 4 years of data.