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Cast iron drain pipes typically last 75 - 100 years before corrosion becomes a problem, so if yours is showing signs of failure like slow drains, backups, or visible rust, you don't necessarily need to replace the whole line at once - a plumber can inspect it with a camera to pinpoint failed sections and replace just those parts, though sometimes it makes sense to do the whole thing if most of it is already compromised.
You don't always have to rip out the whole system at once, but here's the thing - if you're already seeing failures in one spot, the rest of that pipe is probably in similar shape underneath, so patching just one section often means you'll be calling someone back out in a year or two when the next weak point gives up! The smarter move is usually to replace the problem section now and plan for a full replacement down the road when you've got the budget, rather than playing whack-a-mole with repairs.
Don't assume one bad section means you have to replace everything right now - but don't ignore it either, because cast iron degrades from the inside out and what looks okay on the surface is often crumbling underneath. I had a bathroom drain start backing up, got it scoped out, and the inspection showed pinhole corrosion pretty far along the main line, so patching just that one spot would've been throwing money away; the plumber recommended replacing the whole run since the rest was equally compromised even though nothing else was actively failing yet. If you can get a camera inspection (seriously worth the cost), that'll tell you whether you're dealing with localized rust or system-wide deterioration - that's the difference between a targeted fix and a full replacement.
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