One can recap the syringe and save the rest for later. Why would one use a whole syringe for 3 cakes? 1-2cc's is plenty per cake. It is a waste of spores and adding too much water may be detrimental to the cakes health. We don't want the mycelium to be swimming in the cakes.
On average, it takes 4-7 days before mycelium will appear.