I think a 1/2 hour is just about right. That way you can jump in the pool, or relax in the hot tub for a bit , and not lose your spot.
What really chaps me is when people go up on deck at 6am and reserve lounge chairs, and then go back to bed for a couple of hours, eat breakfast in the dining room, then go back to their cabins to change into their swim suits, and finally make it to their lounge chairs at 11am!
I'd really like to see the deck staff enforce a 1/2 hour limit, so that everyone has a fair chance at a lounge chair.
I'd also like to see the deck staff enforce other rules, too. Like unruly kids running around, the parents of said kids, loud obnoxious behaviour, and so on.............
Well, I can dream, can't I............!