As I understand it, what you get in material length depends entirely on what you order. If you just order a traditional kilt to your size, here in Scotland a widely used rule of thumb is that for up to about 30-32ins waist 6yds are used, 30-38ins waist has 7yds, 38-44ins waist has 8yds, and 44ins and over needs 9yds. The size of the sett decides the number and depth of pleats.

However, if you actually specify 8yds then that is what you should get (Trades Description Act, etc etc) and although the kiltmaker will start out with 8yds of material, the finished kilt will not actually measure the full 8yds round the longest edge (the bottom edge, straightening out the pleats) as some 4-9ins can be "lost" in hems, joins etc.