How long to get the receipt

How long can I get the receipt after filing I-140/485? I was told that USCIS will open the package and send back the receipt as soon as they receive the package? I mailed with return receipt and already got the return receipt from USPS, but did not get any information from USCIS( more than 1 week now)? So worrying about it! Can someone share the experience? thanks, thanks