Realistically a mild steel exhaust would take a long time to rust the factory exhaust in my d22 looked hell rusty after 13 years but there was no holes in it. Id say to rust an exhaust out would take a solid 20-30 years, unless of course you live on Fraser island etc, but then the car wouldn't last as long anyway and other things would be more of an issue then a rusty exhaust. Most cars/engines don't even have 20-30yr life span anymore haha. But as everyone says at only $100 extra for stainless it is worth it. Also some people say stainless exhausts have a crisper note to them and sound different to steel.