As of March 2018, Machines Service Centre is able to cut down the battery replacement service to 4 hours (within same day) or the following working day if submitted by 3PM.
Standard repair turnaround time is estimated within 7 working days (excluding weekends and public holidays). Repairs may take longer subjected to part availability
For iPhone 6 Plus and iPhone 6s Plus currently have a longer lead time because this iPhone is no longer being manufactured. Replacement batteries will be available in late March/early April.
All other iPhone models - Parts are expected to be available without extended delays.