RT60 is the standard measurement of room reverberation. Professional mixing rooms target 0.2–0.4 seconds across all frequencies. Untreated bedrooms are typically 0.5–1.5 seconds. Home theaters target 0.3–0.5 seconds. Concert halls: 1.5–2.5 seconds.
Critical detail: RT60 should be roughly equal across all frequencies. A room with short high-frequency decay but long bass decay (common in untreated rooms with thin carpet and foam) sounds simultaneously dead and boomy — the worst of both worlds. Broadband absorption (thick panels) targets all frequencies equally.