Hi Veisley thanks for reaching out. Unfortunately we don't currently have a feature to show you the total work time for a document. If this is something you would find helpful please feel free to promote the idea to our development team using this feature request form. 

As an alternative- if you have access to the feature you could check the revision history and try to calculate the total time spent working on the document based on the versions saved. Please see this article for more information on using revision history.