Predicting the battery consumption will be near impossible as it depends on several factors:
- The device's processing power
- The device's screen size, type and brightness
- The internet connection speed on the device (most speech recognition services send the data to a server)
- The efficiency of the hardware microphone
- Other background processes running on the device
Even if everything was in ideal conditions, the simple fact that different devices have different screens, processors and battery capacity will make it impossible to predict the consumption.