Start your stopwatch, run the procedure and stop it again when it finishes!

But seriously, you just need to record the time it started, the time it finished and find the difference. How you do this depends upon your environment. And you haven't told us what you're using.

In SQL*Plus you can set timing on:

SQL> create or replace procedure p as
  2  begin
  3    dbms_lock.sleep(5);
  4  end;
  5  /

Procedure created.

SQL> set timing on
SQL> exec p;

PL/SQL procedure successfully completed.

Elapsed: 00:00:05.28

Or within PL/SQL you can call dbms_utility.get_time:

SQL> declare
  2    t1 pls_integer;
  3  begin
  4    t1 := dbms_utility.get_time;
  5    p;
  6    dbms_output.put_line((dbms_utility.get_time - t1)/100 || ' seconds');
  7  end;
  8  /
5 seconds

PL/SQL procedure successfully completed.

Or you could log to a table, or use the profiler:

https://oracle-base.com/articles/11g/plsql-hierarchical-profiler-11gr1

If you want more information about a specific environment, you'll have to tell us what you're using.