Kill a campaign when it has spent enough to exit the platform's learning phase and still misses your break-even CPA or CPL target with statistical consistency — not after two bad days. A common framework is roughly two to three times your target CPA in spend with no conversions, or a sustained CPL well above target across a full week of stable delivery.
Watch for confounders first: a frequency spike, a paused audience, a broken landing page, or a tracking issue can masquerade as a genuinely bad campaign. Kill the true underperformers, but rule out fixable causes before pulling the plug.