Once you disconnect, Dish will ship you boxes and labels. Once you receive the boxes, ship back the receivers the same day. Make sure you keep a copy of the tracking numbers just incase Dish misplaces the receiver.
I've heard different timeframes but I had heard 15 days after you receive the boxes from them to return the equipment in. They know when you get the boxes....
I didn't take the chance. I returned the equipment at my own cost the day after cancelling. When they rec'd it, I called and confirmed with them that they'd rec'd it and had the account noted as such.