I.e. how long I’ve spent in the editor, making changes to code etc!
If it’s a static site it is easy.
Just go to the dashboard. (If you’re in the editor, do this by clicking your avatar and then clicking “your projects”)
On the dashboard you’ll see a count of how many project hours you’ve used and how many you have left. (project hours keep track of how much time you’ve edited a project; and if it has a server-side, then how many hours it has been awake [assuming it is not a boosted site])
Those project hours are a record of how much time you’ve spent in the editor unless your situation is the same as the one mentioned below.
If you have any sites that aren’t static, then there is no way of knowing how much time you’ve spent in the editor without knowledge of how many project hours you had used before you went into the editor.
I hope this helps