Set a category default to never expire: this is a feature available for professional level plan users and it applies to only newly entered items.
Navigate to category settings then add or edit a category to display the screen below:
*Please again note, this is for newly entered items in the system, not new (as in new and used) items
With this setting checked when you add inventory under this category the expire date for consignment inventory will be blank:
*You can not move inventory from a category that has this setting turned on to a different category that has it off, then move it back to the original and expect the item to go back to no expire date - only newly created items that stay in the category will never expire.