Hopefully it will arrive fairly quickly. You should receive packet 3, where you have to mail back your DS160 confirmation page, 2 passport photos, etc. Our packet 3 included a copy of our originally mailed petition. I’ve seen letters for packet 3 from different embassies that vary in what they request, so ymmv. After, you successfully complete packet 3, then you will receive packet 4 with instructions to sign up for your interview date, they won’t just assign a date to you, you’ll get to pick.