Time elapsed Calculation
Do you want to calculate it by hand, or is it enough to let the oracle db to do the calculation?


  Question:  I want to see the response time for my SQL statement and see the run time.  What tools does Oracle provide to see how long a query runs?
  
  Answer:  For seeing the elapsed time for an individual query, you can see individual query response time in SQL*Plus with the “set timing on” command.
  
  SQL> set timing on;
  SQL> select stuff from mytab;
  
  Elapsed: 00:00:02.82


(directly from http://www.dba-oracle.com/t_measure_sql_response_time.htm) may be of some interest in that case.