According to State Department regulation all J-1 Exchange visitors’ participants must validate their visa within five days of entering the U.S.
The validation process was created to make sure that all participants arrive in the US safely and contact their sponsor so their visa can be activated. The process of activation can take up to three business days. Please do not apply for a Social Security card until five business days after you validate yourself with your sponsor as it may take that long for the two computer systems to talk with each other.
Failure to validate your visa in a timely manner means that you are in violation of the program and cannot work or obtain a Social Security number. It can also lead the sponsor to terminating your program leading to you being in trouble with the US government.