Far enough ahead that an option requiring an employer decision, new evidence or a change of role could realistically be pursued. Reconstruction alone can take weeks once old records are involved, so record gathering usually needs to begin well before the final year of the current period.
Let the slowest dependency set the start date
Old passports, former employers and previous representatives respond on their own schedule, and an internal sponsoring decision can take a full budget cycle. Identify the slowest link and start there. Set a date for the completed chronology, a second for counsel's reading, and a third for any employer conversation.
Keep travel plans in view, since departures and admissions during this period become part of the record being built. Add one further consideration to that dependency ordering: the employer's own decision cycle is often the slowest link and the least visible. A sponsoring decision may need a budget line, a legal review, a committee that meets quarterly, and an approval from someone who has never encountered the question before, and none of that appears on an immigration timeline.
Ask early what the internal process actually is and who owns each step, then place the external requests alongside it rather than behind it. Hypothetical example: a worker at an analytical instruments maker completes a chronology in six weeks and waits eleven more for an internal decision that nobody had identified as a step, which is a delay entirely within the organisation and entirely predictable had anyone asked at the outset.