The time an Activity takes to process a Work Item is controlled by the timing information on the Activity dialog highlighted by the red box in the picture below.
In distribution/timing panels the parameters of the distribution vary with the type of distribution.
The first box is almost always the MEAN (or expected value) of the time being specified. So you can expect the average amount of time taken by a process to be the value in the first box. (But beware of the labels on the boxes - some distributions do not have MEAN as a parameter - for example: Uniform Distributions).
Click the down-arrow button to see a list of available distributions (select the one you want) or click the NEW button to create a new distribution of your own.
The DETAIL button in timing/distribution panels enables you to check and change the parameters of distribution that you have specially created for your simulation (including named distributions).
The DETAIL button is only enabled if the distribution type selection box is set to a distribution that you have defined.
The TIMING button of the Activity offers a choice of timing options: Standard, use Labels, use Multi-task, use Jobs Matrix, use Cycle Matrix, and Zero. Read more information on timing orientation