The concern with shelf life has to do with the "rubber" suspension parts. Some of these can change in characteristics with age. That tendency increases with the level of pollutants, particularly ozone. If I planned to keep a cartridge for a while, I would consider sealing it in an air-tight container.
That said, I know of a lot of people using 20-30 year old cartridges that are perfectly fine. Koetsu, as a brand, comes to mind. Another brand, Allaerte specifically touts, as a feature of their cartridges, the use of suspension elastomers that are well more than a hundred years old so that their characteristics are now quite stable. I don't know about Benz specifically, but then again, I have not heard anything about their cartridges going prematurely bad, so I bet they can be held for quite some time.
That said, I know of a lot of people using 20-30 year old cartridges that are perfectly fine. Koetsu, as a brand, comes to mind. Another brand, Allaerte specifically touts, as a feature of their cartridges, the use of suspension elastomers that are well more than a hundred years old so that their characteristics are now quite stable. I don't know about Benz specifically, but then again, I have not heard anything about their cartridges going prematurely bad, so I bet they can be held for quite some time.