Create a new Calculated Number Field (by JWT) custom field and name it properly.
Providing a description will help you to identify what the custom field does but this step is optional
Parser expression
Add the following expression:
sum(fieldValue({issue.remainingEstimate}, issuesUnderEpic() UNION subtasks(issuesUnderEpic()))) / 480 # The result, which returns the number of minutes is divided by 480 to return a number of working days.
To achieve this, the following functions are used: