Fifty years of acid, from the inside

Nobody authoritative publishes a number. The Cast Iron Soil Pipe Institute, the trade association for the companies that made the pipe, states no service life for drain pipe anywhere in its Cast Iron Soil Pipe and Fittings Handbook. Its one durability statement is a design requirement reading "durability exceeding expected life of the building", and the hundred year service records it does publish are for buried water and gas distribution mains rather than for the inside of a drain. So the honest answer is that the age of the house tells you to look and a camera tells you what you have. Homes built 1945 to 1975 are the ones worth looking at, because that is when cast iron went in instead of PVC.

Homes built roughly 1945 to 1975 were plumbed with cast iron drain lines rather than PVC. If your house is from that window the drains are the pipe to look at. If it is from 1978 to 1995, the supply pipe is the one to check, because that is the polybutylene era.

Waste water standing in a horizontal run produces sulfuric acid, which eats the bottom of the pipe while the top stays intact. The bore fills with scale, solids catch on the rough wall, and the outside of the pipe looks sound right up until the floor above it does not.

Yes. A camera run down the line shows the inside wall of the pipe, where the scale and the channeling are, and how much of the run is affected. That takes about ten minutes and it is what the quoted scope is measured from.

Usually yes. A drain that clogs on a schedule in the same place is a pipe wall problem rather than a blockage problem. Snaking buys a few months and takes another scrape off a wall that is already thin.

Four, and the footage decides which. A camera inspection on its own if the line is sound. A spot repair if one section has gone. Pipe lining if the remaining wall will hold a liner. Full replacement if the run is past lining.

Yes. The footage is yours, and the quote is built on a measured length of pipe off that footage rather than on an estimate.

Mostly something else, and the pipe industry's own handbook says so. The Cast Iron Soil Pipe Institute's Cast Iron Soil Pipe and Fittings Handbook states that over 95 percent of soils in the United States are non corrosive to cast iron, and names the corrosive exceptions as swamps, peat bogs, tidal marshes and ground contaminated by industrial waste. What ends a residential cast iron drain happens on the inside: the same handbook describes internal corrosion from acids at a pH of 4.3 or lower left in contact with the pipe, which is exactly what standing waste water in a horizontal run produces. That is why the outside of the pipe can look sound while the bore has gone to scale and the bottom of the run is thin.