For ad sets running for longer than 3 days, for example, what period do you use to evaluate them and decide to kill or not? 3 Days? 7 Days?

You use multiple windows to compare the performance and see how it changes over time. If you see it’s getting worse and not profitable anymore, kill it. If you are on a lower budget you give it BE KPI to spend before deciding, if you are in scaling phase and have budgets, give it 2X BE KPI to spend – Facebook Ads KPIs Cheatsheet