A lot hinges on whether you have rasterization going on, complex labeling, highly compressed imagery, etc. in your layers all of which can greatly increase processing time. More hinges on the dpi and the Output image quality. If you are using the Highest try it at normal and 300 dpi. The computer specs are of course a factor. You might Google on ArcGIS pdf performance or something like that for more ideas.
I've got some town by town forest stand types over photos at 36x48 that easily take that long to export to pdf. In contrast the base covertype maps are setup so there is no rasterization of any sort and the labels are all annotext and they zip right out. I've got it set so most of my DDP exports are on my old secondary computer running Arcview 10.1 beta and while the project is exporting I'm doing work on my main computer, but in the past I set things to run overnight on the photo type maps.
