NB-IoT is designed for use cases where end devices need no maintenance for a long period of time. The battery lifetime depends on several factors: power consumption of the radio module, power consumption of the sensor, how often messages are transmitted and the coverage conditions of the end device, among others.
The 3GPP estimated that for a setup with average radio modules and sensors transmitting at a moderate frequency (e.g. 2-3 times a week) in good coverage conditions using two AA cells a battery lifetime of up to 10 years can be expected. However, deviations must be expected in different setups with higher frequency of transmissions i.e. Smart Parking.