Luminance (measured in cd/m2) should be based on ambient conditions around the display and how you'll soft proof the images in relation to prints. So its not possible to necessarily provide specifics here. You want the brightest object in view to be the display so obviously the ambient light plays a role. The lower the luminance on the display, the longer you'll be able to drive that display so while its possible to crank LCDs way, way up (200+ Cd/m2), they are not going to last as long as a lower setting. So, with respect to luminance, YMMV.