I just finished replacing all the seals and hoses on my 1951 1 Ton brake system. I did this because I put it together originally over ten years ago. So here are some details of what I found.
The front wheel cylinders were new when I first put it together. They had a little rust in the middle at the bottom under the inlet but otherwise the bores were in good shape. The rubber seals looked new.
The rear cylinders were rebuilt by a professional rebuilder using stainless steel sleeves. The bores looked good with of course no rust. The seals had started to deteriorate depositing rubber material on the piston face and the sides. So I have a theory that the brake rebuilder shop has a large stock of replacement parts sitting on shelves and some may actually be quite old when used.
Seems like 10 years is a magic number. I have several collector motorcycles that get get little or no use. They seem to deteriorate just sitting and 8-10 years is the sweet spot. I have not tried storing dry or just with brake fluid used during assembly..........
When in regular service the service life is often 20 years.