A formula is the recipe of the Beauty Product. Similar to recipe to bake include the procedure on how to make it.
Short answer at least 3 years. There are tests that the formula can go through that the seller can be confident that their product will last for three years. This would happen once the formula has been approved and before manufacturing.
I will break down the long answer into a few posts.
How long should my beauty product last?
Please comment with you answer.