The 12-Month Bar
Per U.S. Department of State regulations, Exchange Visitors are not eligible for the J-1 Professor or Research Scholar categories if they were physically present in the U.S. in any J status for any part of the 12-month period preceding the proposed program start date for the J-1 Professor or Research Scholar category.
This 12-month bar does not apply to J-1 and/or J-2 participants whose previous presence in J status was less than 6 months, or whose previous presence in J status was as a Short-Term Scholar.
For example, a J-1 student is here for 2 years: August 1, 2014 to July 31, 2016. They then go home on July 31, 2016 ending their J-1 student program. If later on they would like to start a J-1 program in the Professor or Research Scholar categories, the earliest date they would be able to do so is August 1, 2017, 12 months after they left the U.S..