Non-terrestrial networks (NTNs) present significant challenges for reliable communication due to the dynamic nature of their channels. Studying channel coherence time is crucial, since it directly impacts the design of robust transmission schemes (e.g., channel estimation and precoding strategies). This paper evaluates the coherence time of non-terrestrial channels theoretically, revealing that the rapid mobility of non-terrestrial base stations (BSs) substantially reduces channel coherence time. Our results demonstrate that the presence and enhancement of the line-of-sight (LoS) channel play a crucial role in extending coherence time, thereby improving the stability of NTN links. Furthermore, unlike terrestrial networks, where beamwidth adjustments can effectively influence coherence time, our findings indicate that in NTNs, receiver beamwidth has a negligible effect under high-speed BS motion. These insights provide valuable guidelines for designing robust transmission schemes and adaptive signal processing techniques in future NTN deployments.