Place chia seeds and oats in a 4-cup container. Pour almond milk and sweetener, if using, over mixture and stir to combine, making sure to break up any clumps of chia seeds.Cover and refrigerate at least 6 hours or overnight so the mixture has time to thicken. Stir before serving.This will last as long as your almond milk, meaning if you use homemade almond milk, this porridge will only stay fresh a few days.