Brine shrimp can be gut loaded from about the 24 hour point onward in their lives.
Since brine shrimp feed on particulate matter in their water, one can feed them appropriately sized particles of protein, vitamins, or huffas, in order to increase their nutrient value.
Newborn nauplii are rich in huffas, low in protein, loosing their huffas as they feed on their egg sack, but gaining in protein levels as they grow through juvenile stages to adult stages.
For greatest value as nauplii to feed huffa content to your fish, they need to be fed immediately after hatching out. From that point on, they consume the egg sack decreasing the levels of huffas your fish will receive.
It takes about 24 hours to gut load nauplii after they develop mouths at the second instar stage, (about 24 hours old), and less time as they grow, to the point that as adults, they can be gut loaded in not much over an hours time.
Because it takes so long to gut load the nauplii, it's best to gut load them in 12 hour stages, changing the water and food for the last 12 hours. (nutrients degrade quickly so need to be refreshed)
I feed my brine shrimp with Tahitian Blend, a cryopaste mixture, so they can be fed at any time as they are constantly gut loaded with the cryopaste.
Feeding Selco type products increases the Huffa content of the brine, and feeding spirulina increases the protein levels.
What are you feeding brine shrimp to, and what size of brine are you intending to use?