a dry sand from a bag doesn't need to be cured.
the only time you will need to cure sand is if you took live sand and let it dry out or die off. then you'd need to use testing of the curing containers water to see when the ANN spiked and dropped back down.
if your just trying to get bacteria started on the sand it starts the moment you place water in with it, it will build as the food source is increased (your pinch of food). there is no set time till it reaches any particular stage, it just cycles to support what bacteria it can till it runs out of space to colonize or food to feed the bacteria.
hth
kc