Not beyond five years in this classification. The specialized-knowledge maximum is five years in total, so a four-year assignment cannot be completed by someone who has already spent three years in the status, however well the project is going.
The ceiling is arithmetic, not discretion
Do the sum before you accept the assignment rather than in its final year. Hypothetical example: a die-cutting tooling specialist who moved from a Saint-Cyrille-de-Wendover converting plant to the group’s Georgia affiliate is offered a four-year programme with three years of L-1B already behind him; he has two years of headroom, not four, and no extension inside the category will create the rest. Genuine options exist, and each has its own lead time.
Time spent physically outside the United States may be recaptured on documentary evidence, which can recover weeks or months for a frequent traveller. A move into a managerial or executive role, if the duties truly become managerial and the organization can support it, is a different classification with a seven-year maximum, but it must be a real change of function rather than a change of title. A permanent residence process, if the employer intends one, needs to start years ahead.
And the assignment can be re-scoped so that it ends inside the ceiling, with the remainder handled by someone else or from Canada.