File as soon as the technologist has completed one continuous year with the company and the affiliate's line schedule is fixed, allowing for regular processing or premium processing. She cannot start work at the U.S. plant before approval and admission.
The year abroad is the gate, then the processing time
Two clocks run. The first is the continuous year of qualifying employment abroad within the three years before filing; a technologist hired five years ago has long satisfied it, but a recent hire has not, and no urgency at the affiliate changes that. Weeks spent at the U.S.
plant on company business do not interrupt the year, although they are not counted toward it. The second clock is USCIS processing, which can be shortened by premium processing for a fee. A commissioning date in the spring suggests filing in the previous autumn.
Initial L-1B approval can be for up to three years, so an eighteen-month assignment fits inside one petition, and an extension is possible within the five-year maximum. Until approval she may visit the plant as a business visitor to observe or attend meetings but may not set the line, train operators or otherwise perform the job she is being transferred to do.