They vary a lot, as they're not galvanised and wax injected by the factory (which would be nice). To balance things up, Marios reported that my subrames were good in November when he put in a new clutch and serviced it. It's a December 2006 QPV and it's seen a few winters in the south of England, but it's also been pampered and garaged most of its life. When I was looking to buy I found at least one 2007 car which had failed an MoT due to subframe corrosion.
Mine had very light surface rust in places when I reverted the springs to standard in July, so I treated that with Dinitrol externally. Marios kindly wax injected it at the service - something I was thinking would be worthwhile, so I really appreciated him doing it. Wax injection is why my Alfasud is still rock solid, so I really understand the value of doing this.