Everyone has they're idea of life spans of forced air and ac's...15-20 if maintained, etc...how about boilers? Hot water vs. steam and by todays standards....not the 50 year olds that you see...
With the smaller water passages, and thinner casting, if it has the recommended water flow 30 + years my guess.
If low water flow through the boiler, 20 with luck.
three HUGE factors
Longevity on iron and steel boilers is dependant on the quality of the solvent (water) that you use inside them.
First factor, what's the cleanliness of the water that you run in the boiler? Is it full of flux, oils, and pipe dope? Did the boiler and system get cleaned before it was put into service?
Second factor, what is the condition of the water that is used after the system is cleaned?
Third, how tight is the system? If we use hard or contaminated water in a system that has a leak, we are constantly refreshing the boiler water with new contaminants, automaticly.
In summary, we change boilers AFTER they leak, so why do they leak? In general, they leak because the water has corroded the system, or contaminants drop out of the water and plate onto surfaces (causing stress failures), and it's worse where the water is the hottest.
Assuming a quality installation from the start, and a modern CAST IRON name brand boiler too, a hot water boiler should last a minimum 35 yrs........steam, more than 20 yrs. Leaks on steam system wreak havoc, doesn't appear to be a really big issue with hot water boilers though. At least from what I've seen in the past 20 yrs.