Allow time to prepare the new employer's evidence file, complete the correct authorization step, and confirm the result before setting a start date. There is no fixed period, since it depends on the applicant's location, the chosen authorization path, and how quickly the new employer's documentation is ready.
Do not set a start date before authorization is confirmed
Work backward from the earliest date the new authorization could realistically be in hand, not forward from the employer's preferred start date. Ask the new employer whether the role can accommodate a delayed start if preparation or processing takes longer than expected. Avoid giving notice to the current employer until the new authorization step is either completed or firmly scheduled, so there is no gap in lawful work authorization.
Work backward from the earliest date the new authorisation could realistically be in hand rather than forward from the employer's preferred start. There is no fixed period, since it depends on the applicant's location, the chosen path, and how quickly the new employer's documentation is ready, and the last of those is the one most often underestimated. Ask whether the role can accommodate a delayed start if preparation or processing takes longer than expected, and get that answer in writing.
Avoid giving notice to the current employer until the new step is either completed or firmly scheduled, so no gap opens in lawful work authorisation.