@Nageshwari P 

you will get to know the RITM created time, you can get the approval time from sysapproval_approver record

Then find the difference using your schedule (the schedule should exclude holidays, weekends etc

Recently I shared solution for something similar, check that

calculate Duration and Business Duration in scoped application 

also check these links

subtract time from schedule to update task due date 

Calculate Date By Adding or Subtracting Time With A Schedule 

If my response helped please mark it correct and close the thread so that it benefits future readers.