Far enough to see the five-year ceiling. Count any prior time in L status at the outset, document time spent outside the United States as it accrues, and decide before year four what happens at the end. Promotions and extensions both need lead time measured in months, not weeks.
Work from the ceiling backwards
Assignments are usually planned forwards from a start date, which hides the constraint that matters. L-1B is capped at five years in total and prior time counts, so the question that should be answered at the beginning is what happens when the cap is reached. The options are limited and each needs preparation: a move to L-1A, if the managerial test can genuinely be met, which takes its own petition; a different classification altogether; an immigrant petition, which has its own multi-year timetable; or departure.
Deciding among them in the final year is deciding under pressure. Alongside that, keep a contemporaneous record of days spent outside the United States, because recapture of that time is only available if it can be documented, and reconstructing four years of travel from memory is not realistic. Extensions and promotions should each be started several months before they are needed.