AE provides an initial ETA based on a export rate of 5 images per second. Depending on machine performance, the nature of the storage holding the Aperture library, and the destination drive performance the time can vary significantly. Other factors such as image file size, RAW/JPEG/TIFF mixes and the options selected for AE also have a significant influence on export times. The ETA is updated during an export by observing the real rate of exported images and taking into account the real impact of the options selected. As a result, it is impossible to say exactly how long your export will take a priori.