My plumber is all most finnished putting my lines in where I can begain to pour my slab. My question is what depth should these be. They will being running into a septic tank, then out of a field line. My buddy said that they put his pipes just below the forming boards. They are putting mine down 18"s. He said that is to low for my septic tank to work. He just built a new home a few months ago, and I'm not using his guy, so I think he is just naging on things. But I need to know.

It all has to do with elevation. If the house main's highger the septic tank inlet you'll be just fine. Putting the main 18" under the pour isn't excessive, All the buildings in my area are on slabs and our pipes are anywhere from 14 to 18" under the pour.
Cheers, n Tom
I take it this is a slab house with no basement. What's the frost depth in your area?