In order to have a better understanding of the total cycel time one also have to understand the concept of batching, and how it affects the total cycel time. Finding out your cycel time tend to seem easy:
Total Cycel Time = Number of things in Process / Average Completion Rate
The picture tries to illustrate on how the delivery is affected by driving parallel projects (all-at-once) comparing to one at the time (focused). Also, it tried to illustrate what happens when the average completion rate increases and decreases.
However, there are some very interesting phenomenons in batching and cycle times that requires your attention. For example, take a look at the picture below to see what happens once you try to deliver twice as much.
Summary: There is of course situations where (for different reasons) it is very hard, and sometimes impossible, to avoid the situation of driving too many project at the same time. However, one must be very aware of the effects and consequences from context switching, batching, variations, seven wastes etc. To my experience, way too many companies are trying to do way to much at once – instead of focusing, and in that way delivering something with much shorter cycle time!