Calculate Date Durations between 2 dates

Hi icassiem - your suggested method of using a calculated column for individual durations and a calculated measure for aggregation is correct.

using datediff. calculated column calculate duration days:

Duration_Days =
DATEDIFF(
TableName[StartDate],
TableName[EndDate],
DAY
)

create a measure for total duration as 

Total_Duration =
SUM(TableName[Duration_Days])

Convert Duration to Hours or Mixed Format

Duration_Format =
VAR Days = INT(TableName[Duration_Hours] / 24)
VAR Hours = MOD(TableName[Duration_Hours], 24)
RETURN
Days & " days " & Hours & " hours"

In your visual e.g., a table or matrix, add the "Stage" field to the rows.Add the Total_Duration measure to display the summed duration per stage.