I electronically filed my provisional patent application on December 26, 2018. When can I expect to receive my provisional patent filing receipt from the USPTO in the mail? At what point can I began to use the term "patent pending"?
For applications filed electronically, filing acknowledgement and fee receipt is generated immediately. Option for print out is given immediately. If you have missed it than you can get the same from from EFS-Web PAIR. As about the receipt being sent by USPS mail, US residents may be getting it, but overseas inventors don't (at least for an pro se inventor from India). If you are registered e-filer, you can give email option for notification and will get notification.
The filing receipt in the mail on January 7. It was mailed on January 4, so it took approximately 8 days for the application to be processed, and 11 days to reach me.