If it's been chemically treated properly, flushed and cleaned on initial fill, and the system is flushed and cleaned every few years, and the pipe velocities are below 8 feet per second on the big pipes and less than 6 feet per second on the smaller pipes, then one should expect at least 75 years of useful service life, and even longer. I've seen heating pipe that came out of a 1910 vintage building here that we did renovations on, and it was till in service and working fine.