Use:
set serveroutput on
variable n number
exec :n := dbms_utility.get_time;
select ......
exec dbms_output.put_line( (dbms_utility.get_time-:n)/100) || ' seconds....' );
Or possibly:
SET TIMING ON;
-- do stuff
SET TIMING OFF;
...to get the hundredths of seconds that elapsed.
In either case, time elapsed can be impacted by server load/etc.
Reference: