, there is a considerable difference between a video production application used for consecutive hours at a desk, and an application used to check the weather for 30 seconds on the bus. The former allows for a more robust workflow and the latter requires fast and accommodating tasks.
Designing for the duration of use may seem obvious, but also consider the amount of attention the user is giving to the application. Consider what the user is physically engaged in at the time of use- is the user relaxing? On the go? Waiting in line? The ability to recover from distractions is an imperative component of good mobile application design