• Don't trust the name. A product called "2000mAh" might actually be 1800mAh. Check the manual or the battery itself.
• Account for efficiency. No battery converts 100% of its stored energy into vapor. Most lose 10–20% to heat. For a conservative estimate, multiply your result by 0.85.
• Consider vaping habits. Runtime is continuous use. If your customer takes a 3-second puff every 5 minutes, multiply the continuous runtime by 20 to get a realistic “all day” number.