None. UA is generally instantaneous, both as to e-ticket confirmation and BP's. Only exception is if there is OA travel booked as that can take up to 72 hours (but that is intentional and up to the other carriers, not UA).

If you are experiencing problems with e-ticket receipts, go online and see if you can pull up the record and find your e-ticket receipt that way. Print that. But, until you have an e-ticket receipt, you are right to be concerned.