How much sales history do I need before Prophet is usable?

The honest minimum is 12 weeks of daily sales data per SKU, that’s enough for Prophet to lock onto a weekly pattern and produce a usable 4-6 week forecast. With 26 weeks you start getting yearly seasonality. With 52+ weeks (one full annual cycle including your peak season) Prophet really shines and you can forecast 90 days out with sub-15% MAPE on most retail SKUs. Below 12 weeks, do NOT use Prophet, fall back to a 28-day moving average plus a manual safety-stock multiplier. The model will overfit garbage if you feed it too little.