It depends... FDA says 3-5 days, the mfg says 5-7, but both say to give it at the 0.006% level for 1-2 weeks after treatment.
FDA recommendations:
http://www.accessdata.fda.gov/scripts/animaldrugsatfda/details.cfm?dn=013-149
"Chickens
Indications: For the treatment of coccidiosis.
Amount: Administer at the 0.012 percent level in drinking water as soon as coccidiosis is diagnosed and continue for 3 to 5 days (in severe outbreaks, give amprolium at the 0.024 percent level); continue with 0.006 percent amprolium-medicated water for an additional 1 to 2 weeks."
And this link has these instructions:
http://www.drugs.com/vet/amprol-9-6-solution-can.html
"Poultry - as Soon As Caecal Coccidiosis Is Diagnosed, Give 0.024% Amprolium In The Drinking Water For 5 To 7 Days. Continue The Treatment With 0.006% Amprolium Medicated Water For An Additional One To Two Weeks. No Other Source Of Drinking Water Should Be Available To The Birds During This Time."
The 0.006% dose for Corid Powder is
1/3 teaspoon per gallon.
The 0.006% dose for Corid liquid is 1/2 teaspoon.
The moderate outbreak dose (0.012%) for Corid Powder is
3/4 teaspoon per gallon.
The moderate outbreak dose (0.012%) for Corid liquid is 1 teaspoon per gallon.
The severe outbreak dose (0.024%) for Corid Powder is
1.5 teaspoons per gallon.
The severe outbreak dose (0.024%) for Corid liquid is 2 teaspoon per gallon.
-Kathy