I define "duration anxiety" as the anxiety one feels when the duration of an activity is longer than one feels it should be, which in turn causes more anxiety. It's a sort of anxious feedback loop. The best way to describe the idea of duration anxiety is with an example.
Consider the act of falling asleep. I generally fall asleep reasonably quickly, within 10 or 20 minutes of turning out my light and laying down. However, there are rare times when it takes considerably longer. Perhaps I consumed just a little too much coffee that day and it's still in my system. Maybe I have some pressing thoughts on my mind that won't be quieted. Or my body may simply not be tired enough from the day's activities. Whatever the reason, I am left in the uncomfortable state of waiting to fall asleep.
I generally don't like waiting. I doubt most people really do. Waiting is almost by definition a waste of time, because one is simply idling until something happens. I cannot consciously choose to fall asleep, my body must do that itself. I don't have any explicit control over the matter. There is little I can do but wait in the darkness of my bedroom. This is where duration anxiety comes in.
The combination of the powerlessness over when I fall asleep, and my knowing that the longer it takes to do so, the more time I waste and the more tired I will be the next day, is very frustrating. The problem compounds itself because my frustration makes it even more difficult to sleep. And so I find myself stuck in a positive feedback loop. This is the essence of duration anxiety: excess duration resulting in a positive feedback loop that only serves to lengthen the duration further. Luckily, in the example of waiting to fall asleep, I do inevitably end up falling asleep eventually once my body is just so overwhelmingly tired that it supersedes the frustration I have.
The solution is probably to simply relax, knowing that frustration only makes things worse and that the body will sort itself out if given time. But this is much easier said than done. In order to do so, I've found that a good strategy is to simply do another activity for a while to distract oneself from your current plight. This gives the body time to relax and allows the anxiety to dissipate.