Using the dashboard , each user has a list of the items he/she should work on.
Once the the work is 'done', the list retains these items for a configurable number of days (by default 180).
This is configurable through the 'user settings'
In the 'GUI' tab set the value for 'Max Age (in days) of completed items in the dashboard (0= show only active items)' to the number of days you desire
In the below example, the 'done items' will remain visible for 10 days.
Setting this to 0 would make them disappear immediately.