Shelf life on these old paper units is essentially forever, or until the case rusts through. This is a paper dielectric capacitor, and most likely the actual insulation is provided by oil. Only issue is the oil will be a PCB compound, so if you smell oil on the unit, or it is leaking oil then you need to handle as hazardous material. If it is not leaking it will be fine, I have never seen these fail other than from physical damage or dead seals, unless you apply more than double the rated voltage ( done for 30s during manufacture) for an extended time, or drive high current through it ( I have a 0.1uF 2kV mica capacitor that is rated for 11A RMS at 30MHz) to overheat it. These were normally used on high power valve transmitters, and are still used, even if the units are getting close to 50 years old, they rarely give problems.