If you're reunning cable through the wall, who cares how thick it is, you have to drill all the way trough anyway. But, unless you live in a house built abnormally, it has 2x4s as studs (2x6s in rare cases). Plus it has 1/2" or 3/4" drywall on either side. So... 3.5" + 1/2" + 1/2" = 4.5". Or if you have thick drywall, then it's 5".
Or, just drill a small hole all the way through and measure if with wire or something. THen putty the hole back up.