There's no fixed period; it depends on how quickly the employer can produce an accurate duty description and how quickly any licensing gap can be closed. Build the schedule around those two dependencies rather than a general processing estimate.
Track the employer and the licence separately
Set a date for the employer to deliver a revised duty description if needed, and a separate date for confirming licence or registration status. These often move at different speeds, and a case can stall on one while the other is ready. Only after both are settled should travel and admission planning proceed.
An employer pushing for an early start date should be told plainly that the mismatch needs resolving first, since filing on an inaccurate duty description creates larger problems later. Distinguish the items the applicant controls from those they do not. A revised duty description waits on an employer's internal approvals; a registration confirmation waits on a regulator's queue; and where a petition route is used, processing waits on nobody in the room.
Put each on the schedule with its owner named, and expect any item whose owner is external to be the one that moves. Hypothetical example: a land surveyor's file is complete except for a licence verification the regulator issues in batches once a month, so the realistic start date is set by that cycle rather than by the applicant's readiness. Where an employer presses for something earlier, the honest response is to show the cycle rather than to compress the estimate and hope.