Request it as soon as the offer is real, because regulators set their own processing times and a letter with a short validity window can expire before it is used. Work backwards from the intended start date rather than forwards from today.
Let the slowest issuer set the schedule
Build the schedule from the outside in. Fix the intended start date, then ask each issuer how long it takes and whether its document carries an expiry. A regulator's certificate of standing is often dated and treated as current only for a limited period, so requesting it too early is as unhelpful as requesting it too late. Transcripts sent institution to institution are frequently the slowest item, particularly across a summer. Degree equivalency assessments add their own weeks. Only after those are placed on the calendar does it make sense to choose between a port-of-entry request and an employer petition, because the two have different preparation windows. If the employer needs the position filled before the slowest document can arrive, that is a fact to raise with the employer, not a reason to present the file incomplete.
Hypothetical example: A college instructor receives an offer for a semester beginning in late August, but the registrar will release an official transcript only after final grades post. The first review should create a backward calendar from the reporting date and identify the slowest issuer. A signed support letter cannot replace an unavailable transcript, and rushing to an inspection location with a partial file may end the planned start rather than preserve it. Confirm whether the employer will use the USCIS petition route or an arrival-based request only after the key records are obtainable. Leave time to correct a title, obtain a fresh licence verification, and arrange family travel separately.