I spend a lot of time at the pool. I swim a lot, read, talk. On sea days, I rarely leave the pool area except for a trip to the lunch buffet, which only takes about 20 minutes, at most.
I think the 40 minute rule is a good one, if it can be enforced. I've seen too many chairs "reserved" with books or towels for hours. If someone new comes looking for a chair, I usually tell them, "No one has been there in hours". It's up to them. whether or not to remove the items.
In our resort town, the police "mark" tires with chalk to enforce 3 or 4 hour parking limits in public lots. Why not mark pool chairs?