Before you can predict runtime, you need three numbers:

• Battery capacity in milliampere-hours (mAh). This tells you how much charge the battery holds. A higher number means more stored energy.
• Power draw in watts (W). This is how fast the device uses energy. Higher wattage = faster drain.
• Voltage in volts (V). Most regulated mods use a nominal voltage around 3.7V, but check the spec sheet.

If you're looking at a device like the VERTEX-VISTA or LINQ-PRO, the spec sheet usually lists the mAh. For the power draw, check the wattage range. If it's adjustable, use the average you expect your customer to use.