Narrow-Band Internet of Things (NB-IoT) is 3GPP's cellular technology designed for narrow-band and Low-Power Wide Area Network (LPWN). NB-IoT provides deep indoor coverage for thousands of low-data-rate and low-powered devices. Coverage enhancement for devices in weak coverage condition is enabled by having signal repetitions over extended period of time in order to boost the signal quality. Performance gain from repetitions of signals is limited by channel estimation quality. In this paper we analyze impact of channel coherence time on the uplink coverage. In addition to analytical performance bound and simulation results, we also present coverage performance results from practical measurements using a NB-IoT prototype. It is reported how the NB-IoT coverage improvement is limited by the channel estimation quality and coherence time of the channel.