We applied for a stay extension for my parents who are currently in USA on B1/B2 visitor visa. We applied for i539 for stay extension by few months. The application was applied via post and received by USCIS on Feb16 2022. We didn't receive any receipt notice or acknowledgment and had following questions.
Q) What is the average wait time to receive a receipt notice?
Q) Their i94 is valid till Mar 20 2022, can they stay if the receipt notice is not received? We have mail proof that package is delivered to USCIS from UPS
Q) When can we follow up with USCIS about the case, about receipt?