Set a consignor default to never expire: this applies to only newly entered items.
Navigate to the consignment tab then add or edit a consignor to display the screen below:
With this setting checked when you add inventory under this consignor the expire date for consignment inventory will be blank:
*You can not move inventory from a consignor that has this setting turned on to a different consignor 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 tied to the consignor will never expire.