Well before the first one approaches the ceiling. Both the qualifying year of employment abroad and the acquisition of internal knowledge take real time and must be documented as they occur.
The replacement takes years, not months
Companies tend to think of succession as a staffing question and discover it is a timing question. A second employee who could later satisfy the requirements needs a continuous qualifying year with the organisation abroad within the three years before any future petition, and needs genuinely to have acquired the company's internal knowledge, which cannot be conferred by a memo. Both have to be documented as they happen.
Starting that process in the fourth year of a five-year placement leaves no room. The better question, asked at the beginning, is which phases of the U.S. work truly require a transferred specialist and which can be handled by local staff trained to the company's standard, since developing that local capability is itself the most durable answer to the ceiling.