this depends on where you live and how cold the winter gets.

in places you don't have to worry about frozen earth, the sewage can be as shallow as a couple of feet. It's not a big job to dig up and replace. But in cold places the sewage has to be deep enough to avoid freezing. For example, I had a couple of digs that went to 8 ft below...Now that's more than a permit from the town or city, this gets into the OSHA regulation territory...