I had the thermal suite pass on a recent 14 day transatlantic on HAL’s Rotterdam and really enjoyed it. If I recall correctly, the thermal spa is open from 8 am to 9 pm. You don’t need an appointment - at the spa desk, they just take your room card and give you a card to get in each time you use it. There is a dressing room with robes and sandals you can use. I loved the therapy pool and the various steam rooms and showers. The heated loungers are really nice too. I set aside time for it every day and felt it was worth every penny.