No stamps are mailed for efile. Print it, you will not receive an actual stamp.
For paper forms, it's common to go 30 days without receiving a stamp. After the 30 days are up and you have still not received your stamp, you can request a certified copy.
They are currently mailing out approved paper forms in batches so your stamp may or may not be mailed close to the time it was approved. It could mail out the next day or it could sit in the stack of approved forms for up to 2 weeks.
They're required by law to actually affix a real stamp to a real paper, and send it to you. Surprise, surprise, they don't follow the law when they don't want too!
For eFile forms, they just upload a digital image on your online form, but don't tell you or let you know that you print that as your approval.