Currently, I am analysing cases and the time spent on each one. I know what is an acceptable time, but I am experiencing a difficulty to decide what is are the outliers in the range of data (some are quite visible).
Have you ever used a formula and a rule what deviation should be used in order to define the outliers?
be careful about moving "outliers" as this can easily distort your interpretation of the results.
There are statistical methods as others have commented.
However, before removing an outlier you really should look at that occurrence and try to find out WHY it is an outlier - why is this observation so different from the others? If there are genuine reasons for the difference then remove it If there aren't then you should leave it in the data set as it represents a real situation
You could also think about using quality control charts to monitor your data over time. These set pre-determined limits for outliers