I sent mine in at the beginning of June (got proof of posting) and still haven't had anything back. Eight weeks so far.
Last year, the same thing happened and I got so anxious that they hadn't received it before the deadline, that I went online and re-submitted it. That made things worse - my whole claim was closed down. Apparently their computer system can't cope with getting the same information twice.
I renewed my tax credits in June by form. Just checked my bank account and I have a new payment and it's much more than I have been getting. Am worried I have been over paid now as I didn't get any for about 5 years!