For a proper answer you'd have to look up the plastic type and check for conditions where it would degrade. Plastics vary a lot by type and conditions of storage and exposure to sunlight.

As an example you could probably keep a container of polypropylene (code 5) or HDPE (code 2) with salt for at least 5 years in a dry dark place without any concern. Salt can still scratch the outside of the container and cause minor plastic pollution if shaken every now and then for 5 years.

However, if you want to make the salt last for your whole life then a glass/ceramic/stainless steel containers are the way to go. The life of the salt would then be mostly limited by moisture in the air so if you manage to make a design of the lid to allow airflow around a package of silica or rice you'll have your forever container.