There is no single answer. Depends what lenses you use, and how you use them. I routinely keep a 24 inch rail on even my 4X5 Sinar because I like to work with long focal lengths. But it's comprised of a 12 inch center section and then a 6 inch extension at both ends, so it takes mere seconds to shorten the rail if needed. When it comes to 8x10 format, the longest lens I use is a 600mm Fuji C, so 750mm of extension is ample, since I use shorter focal lengths for close-ups anyway, especially a 360 Fuji A.