The USA transit visa is generally issued immediately after checking your credentials. However, due to the heavy, constant demand for visas, a minimum of five working days is normally required for the issuance of a crew-list visa.
Note: It is advisable to contact the American Consulate near your residence for C-1 visa so that your visa would be mailed to you in advance or it would be ready for you at the airport/seaport.
You need to submit a filled in USCIS Form DS-156 along with:
Note: The consular officer may require additional documents to verify the purpose for obtaining the visa.