Well I went in and made some bookings and learned a bit. Only 4 free nights per stay. Checking the terms of 4NF:
5th Night Free: For every Standard Room Reward stay of five or more nights, every 5th night is free. Members can utilize the 5th Night Free benefit on an unlimited number of stays annually. Applies only to five consecutive nights within the same full Points Standard Room Reward stay at the same property, up to 20 nights, when confirmed at time of booking. No more than four free nights will be awarded per stay.
Indeed when I try to price out 30 nights, it stops adding free nights after the 4th one.
Anyone had any experience with this? I am not keen to have to check out every 25'ish days just to keep my free nights coming but suspect that's what I will have to do. I have a feeling I can book 25 night stays to my hearts delight that show free nights but the system or overzealous office staff may catch on.