Nobody can know. You are not providing any system info or details about to which format you are exporting on what drives using what settings based on what timeline/ footage. You need to provide a lot more info. In any case, ask in the Premiere Pro forum.
Moving this discussion to the Premiere Pro CS5, CS5.5, and CS6 forum. 123rqewqre which version of Premiere Pro are you using? Which operating system do you have installed?