Are you asking how far back you should mount the light or how far back the lower panel, that typically hold the speakers, and the top panel, how far back they should be from the front edge of the side panels?
For the first, it really depends on what type of lighting you use and whether or not your marque will have a diffuser panel behind the picture. I usually use a diffuser and two rows of led light strips. For a standard marque size (around 7" tall), I can get away with about 5-6" of depth. For smaller marquees (like a bartop), I can shorten that to 2-3".
For the second question. I would agree with the previous poster's answer. As far back as necessary so that a marquee retainer (like the one linked) doesn't stick out further than the edge of the side panels. Now if you use aluminum L channel as your marque retainer then I like to match the depth to that of the edge of the side panels to the glass/screen.