Early enough that USCIS approves it at least six months before the five-year limit; with eighteen months remaining, filing within the next few months with premium processing leaves margin. Waiting until the last year risks losing the seven-year limit.
Approval, not filing, must beat the six-month mark
The policy is framed around the approval date: the L-1A classification must be approved with at least six months of L-1B time remaining for the seven-year limit to become available. Because the approval date matters, the petition should be filed with margin using current USCIS processing information; premium processing can provide the published adjudication window. In the hypothetical, filing now, at three and a half years used, leaves roughly a year of buffer.
The organizational change itself should be real on the filing date: the supervisors who will report to her should already be in post or hired under a documented plan, because the petition describes a current managerial position rather than a future one. Subsequent extensions run in two-year increments to the seven-year total, counting the L-1B years already used. A multinational-manager immigrant petition should be considered only if she independently satisfies that category’s qualifying managerial or executive employment-abroad requirement; the change to L-1A alone is insufficient.