The amount of years that a sewer pipe should last varies on the type of sewer pipe you're asking about.
Clay Sewer Pipe is expected to last from 50 to 60 years before cracking because of roots and other things in the soil and needing replaced.
PVC Sewer Pipe as long as roots don't break the sewer line then the PVC Sewer Pipe should easily last 100 years or more.
That means that if you replace an old Clay or other Sewer Line with PVC sewer pipe then it should easily outlast you and likely your Grand Children as long as Roots don't make there way too the sewer pipe and break it.