I might be a bit naive....Throughout this thread there appears to be an assumption that it is ok to reserve a lounger for a period of 20/30/60 minutes while the occupier is off doing something else.
My view is that only time a seat should be reserved is while the occupier remains in the pool area. That means the person is either sitting on the lounger, taking a dip in the pool or getting a drink from the poolside bar.
If leaving for any other purpose (eg a meal, returning to your room) the chair should be vacated.
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The fundamental flaw with the presumption that 20/30/60 minutes is OK is that a fellow guest arriving at the pool has absolutely no idea how long a particular chair has already been vacant for.
When on holiday I don't think anyone has a desire to stand around the deck area for 30 minutes of so waiting to see if an unoccupied lounger becomes vacant.