Seek advice before the leave begins if there is still time, and immediately after if there is not. Options tend to narrow as time passes, and some depend on acting while the current authorised period is still running. Waiting for payroll to close is waiting too long.
Act while the options are still open
Note the end date of the current authorised period and work backwards from it, because several possible steps have to happen before it passes. Ask counsel which of them is time-sensitive on these facts, and diarise the deadline. If the employer is vague about the return date, treat that vagueness as a reason to get advice sooner rather than as a reason to wait and see.
Write the controlling dates on one page: the end of the current authorised period, the leave start, the expected return, and any date the employer has fixed for a decision about the position. That page is the agenda for the first conversation with counsel, and it usually shows immediately which item is the constraint. Where the employer will not commit to a return date, record the absence of a date as a fact rather than leaving a blank, because an open-ended leave is analysed differently from one with a fixed end.
Hypothetical example: a precision-instrument repair business schedules a staffing decision after the authorised period ends, and the mismatch is only visible once both dates sit on the same page.