They'll eventually eat it all up or it will dry up and not be worth eating. Either way, after about 8 hours they have probably gotten all the good they will out of it.
I used it on 2 out of three shipments (one came a week early). I love the stuff. I used a full packet on the 25 ones that were lost in the mail. The survivors had it gone within 24 hours. The last shipment of 31 was on time and I mixed 1/2 of the package with some feed and it was gone within 6 hours.