Mine took 2 months a year ago. My Brother just got one for his new GMC last week. Think it took a little over a month, roof rack another week or two. 2 things to watch. Not sure now, but prior body styles Chevrolet and GMC were different widths at the rear of the box. If you are thinking of painting. Get a rough estimate. It costs more than you probably think. Urethane installed windows are a pain to work around. A long shot but call dealers and car lots.
They often remove them to sell trucks. Sometimes selling to one place for resale. Or if you find a truck for sale with a topper. Leave your number, incase buyer would want the truck without it. Also, maybe a salvage yard. May find one with slight damage if you are going to paint anyway. Used is a pain with so many box choices. Used to be. 6 or 8′, Dodge would fit a Ford. But Chevy was a little different.