With no inhibitor, it depends when they were purchased.
If they're over 40 years old, they'll last pretty much forever.
At 35-40 years old, they seem to last about 25 years.
As they get more recent (and thinner), radiators purchased
10 years ago lasted only 5 years.

This is based on my parents' 50 year old system which has been
extended a number of times over the years, and never had any
inhibitor until just recently. None of the original panel
raditors have shown any sign of starting to leak (their only
problem is they look rather ugly compared with the more recent
ones).

--
Andrew Gabriel