What kind of diving are you doing?
The essence of the DIR setup is a) donate the reg you are breathing, and b) keep your backup bungied under your chin for yourself.
a) can be accomplished with a number of different hose lengths. A 36" octo hose will allow you to breathe the primary and donate it, but is difficult to route because it's not long enough to pass under the arm. A 40" hose will go under the arm, and therefore improve streamlining, but you may need an angled adapter to allow the reg to sit comfortably in your mouth. A 5' hose will route under the arm, across the chest, and around the back of the neck, but may be too short for people with broad chests. A 6' hose may be better for that routing for such folks. A 7' hose will route down to the waistband, up across the chest, and around the back of the neck -- but routes best when using a simple harness with something attached to it that allows capturing the hose underneath it, like a light canister, light, or pocket.
For the bungied backup, the ideal positioning is low enough that you don't bump into it with your chin, but high enough so you can pick it up without your hands (I don't think many of us can do that, actually). Hose length has to be enough to allow you to use the regulator and look to your left. For most single tank setups, 22" is the absolute minimum, and some regulators (and people) need 24". Since the 24" hose is the standard primary regulator hose, I think it unlikely that anyone would need one longer than that.
Lynne (who is Radish today, because ScubaBoard decided not to accept her TSandM login any more)