Sum timestamps IF statements in SQL

I have a problem with summing up the timestamps for only work in progress and was looking for guidance. Attached below is an image from a queried table from google big query. What I am trying to do is basically sum up only the timestamps that are In Progress. What I currently have is a DIFF statement between the minimum In progress time and closed time to get total work time unfortunately that does not give me an accurate amount of time worked because some projects move in and out of to do. What I think I have to do is some sort of loop to find the differences between the in-progress times, but I am stuck on how to do so. Big query table