I would not be comfortable much less than 40" deep.  That is a little excessive, but I don't want to be trying to repair it when it freezes that deep.  A local plumber told me that 3/4" black plastic is just job security for him.

One caveat about brass.  If you are someplace that has ever had coal cinders do not use brass in the ground.  In an extreme case we had a customer whose drive had a fly ash base.  Brass fittings would decompose to the point of leaking propane in 24 hours.