I need to know about the user how much time spends from entry page to exit page.
How to achieve this in calculated metrics ?
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There should be an OOTB metric called "Average Time on Site" that should represent what you are looking for. This metric is in seconds, so you may have to create a calculated metric to change the units.
I also want to add a caveat that if visitors are idle for more than 30 minutes, the session resets to a new one, so you may want to consider that when looking at the upper range of the time on site.
I agree with @kayawalton, the out of the box solution will give you the session length... if the user leaves for more than 30 minutes, a new session/visit will start, and each visit will have it's own session time that will contribute to the average.
In order to measure the time spent from an entry page to an exit page using calculated metrics, you can use “Exit Page Timestamp” and “Entry Page Timestamp” for each user session.
Metric Type: Time
Formula: [Exit Page Timestamp] - [Entry Page Timestamp]
The "Exit Page Timestamp" and "Entry Page Timestamp" should be specific eVars or props in your Adobe Analytics setup that capture the timestamp of when a user accesses those pages.