How often do I need to retrain the model?

Weekly retraining is the sweet spot for retail Magento stores. Schedule a Sunday night cron that fits the model on the last 18 months of data and writes 90-day forecasts to panth_forecast. Monthly is too slow, you miss real demand shifts. Daily is overkill for most SKUs and burns CPU pointlessly. Two exceptions where you retrain immediately: (1) a 30%+ spike in rolling 14-day MAPE (model drift detected), and (2) any time you launch a new SKU, kill an old one, or run a flash-promo that wasn’t in the holiday regressor. Treat those as “intervention events” that justify an unscheduled refit.