Depends upon the changes made to the hardware from previous models. The latest new systems we deployed, the changes to the image took about a 16 hours to complete and test. Normally it takes about 4-8 but the latest processors and BIOS required a certain HAL to be installed to boot correctly. But times will vary depending upon what you have to update. My images are only updated for hardware, software and updates are handled via SMS, WSUS, or scripted installs depending upon the system final config.