Long full scan hasn't returned but session is gone from V$SESSION
It seems you didn't reference the /*+ FULL(t) */ hint alias while calling target table. You created the table with the same name as the alias. I'm not sure if that might be the problem... But the docs example follow this structure:

SELECT /*+ FULL(e) */ employee_id, last_name
  FROM employees e 
  WHERE last_name LIKE :b1;

So, what happens if you instead try:

SELECT /*+ full(t) */  Count(1) n
FROM t t WHERE start_time BETWEEN '2015-07-01 00:00:00' AND '2015-07-31 23:59:59'

?