A useful custom format capability in Excel is the [h]:mm:ss which allows you to see how many hours in total have elapsed between two dates by showing hours bigger than 24 (most time formats automatically change back to 0 so you never see hours of more than 24).
All the other time formats answer the questions what is the time (look at a watch) but this format tells you how long something has been running (like using a stopwatch).
However [h]:mm:ss does not exist for PowerPivot so if you need to see this type of format in the PowerPivot windows you will need to create it using some calculations to convert the number into hours, minutes and seconds where necessary.
But keep in mind that the format does exist in Excel. So when the data comes out of PowerPivot and appears in the Pivot tables in Excel, you can use the correct format to see how long something has taken. You just need to right click on the data, click VALUE FIELD SETTINGS and then Number Format. Choose the Custom options and then find the [h]:mm:ss format.