How do I Sum a column of duration

You are thinking of Airtable as a spreadsheet rather than a database. It wouldn't make sense to sum all of the durations in the same table. Generally when groups of some quantity (or duration in your case) need to be summed, you would have a 2nd separate table and all of the records that have the quantity that require summing would be linked to a record in that 2nd table. You would then use a Roll Up field type to sum those quantities in the 2nd table.