I usually receive them four to six weeks before the cruise, unless it's a last minute booking.
If you don't receive them in the next few days, you might call Carnival just to confirm you're booked on the cruise.
Although it's exciting to get your docs in the mail, don't worry if they tell you everything's set but you still never receive them - you can pick them up at the pier. There's a desk for just that purpose. A lot of people book at the last minute and don't receive their docs in the mail and have to pick them up at the port.
I would definately call Carnival. We sail on the Destiny April18th and never received our docs. I called Carnival a week ago and they told me that they had sent them out and they were either lost in the mail or stolen. Carnival said that they had reissued our docs for pier pick-up and would not do anything else for us.
we booked through orbitz on the 6th for the 26th cruise. we got our docs on the 14th. which was a week later and a week and half prior to departure. a couple of days prior to the 14th, we got a passanger invoice and confirmation that states on the back that says "normally, for reservations made more than 10 days in advance of departure, you will receive your travel documents approximately 7 days prior to departure. " so they still have another week to send your docs. you should get them next week by thursday. if you don't, you should definately call.