Cabanas can be taken out by 6am, I've been catching CX138 normally arrive HKG around 5am a lot, there are few times that I only arrived at Cabanas at 5:35 and there were 3 people ahead of me already.
I used to use the little sofa + chair for a nap a lot and normally set my alarm to 7am. I'm not sure how other people uses the cabanas but I would think no one would go there for just a quick shower, so average use of one hour maybe.