Glanceability and low access time are arguably the key assets of a smartwatch. Smartwatches are designed for, and excel at micro-interactions- simple tasks that only take seconds to complete. However, if a user desires to transition to a task requiring sustained usage, we show that there are additional factors that prevent possible longer usage of the smartwatch. In this paper, we conduct a study with 18 participants to empirically demonstrate that interacting with the smartwatch on the wrist leads to fatigue after only a few minutes. In our study, users performed three tasks in two different poses while using a smartwatch. We demonstrate that only after three minutes of use, the change in perceived exertion of the user was anchored as "somewhat strong" on the Borg CR10 survey scale. These results place an upper bound for smartwatch usage that needs to be considered in application and interaction design.