When a card is moved to the done list in trello, a row is added to a google drive spreadsheet. Use this in combination with a recipe to add new cards in the to do list in trello to a new row in a google drive spreadsheet to be able to calculate how long each task takes you.
Then
Add row to spreadsheet
This action will add a single row to the bottom of the first worksheet of a spreadsheet you specify. Note: a new spreadsheet is created after 2000 rows.
Explore more great ways to automate Google Sheets and Trello