Calculating time spent

I encountered upon an issue while building queries.

My case - I have added extra custom variables for tracking . Now, I want to track time spent on those custom variables.

Doubts -
DB documentation shows - visit_total_time , ----- i m unable to find it in piwik_log_visit
There is a new column named time_spent but there is no mention of it in the DB documentation.

How do i solve this?

Please guide thanks