If you only need to display this data within this table on a report, you can create a new formula duration field so all records have this, or create a report formula duration field just for the report, and then use a filter for those records that are 'completed'.
If you need to do more with this data, you can consider a summary table (then use summary fields with conditions, and create a formula field to perform the average from these summary fields), or on this table create formula fields where you're using formula queries (complicated and dependent on size of records could cause performance issues) to obtain counts of those with a completed status - which will be easy if you also have a formula field dedicated to this.
It looks like you might have a formula field that is doing both things: status calc, and days calc. It might be best to have three fields in total: 1. calc status, 2. calc days, 3. display what you currently have for both (in your image).
Example using a duration field table level:
