Hello @Basu212, Take a look at the below thread:
https://experienceleaguecommunities.adobe.com/t5/adobe-analytics-blogs/sample-sql-queries-to-query-adobe-analytics-data-feeds/ba-p/588992
7. To calculate time spent
SELECT CONCAT(post_visid_high, post_visid_low, visit_num, visit_start_time_gmt) as ...