Here are the paragraphs from the CRB to which I am referring:
Wizard Casting:
When preparing spells for the day, a wizard can leave some of these spell slots open. Later during that day, he can repeat the preparation process as often as he likes, time and circumstances permitting. During these extra sessions of preparation, the wizard can fill these unused spell slots. He cannot, however, abandon a previously prepared spell to replace it with another one or fill a slot that is empty because he has cast a spell in the meantime. That sort of preparation requires a mind fresh from rest. Like the first session of the day, this preparation takes at least 15 minutes, and it takes longer if the wizard prepares more than one-quarter of his spells.
Cleric Casting:
When preparing spells for the day, a cleric can leave some of her spell slots open. Later during that day, she can repeat the preparation process as often as she likes. During these extra sessions of preparation, she can fill these unused spell slots. She cannot, however, abandon a previously prepared spell to replace it with another one or fill a slot that is empty because she has cast a spell in the meantime. Like the first session of the day, this preparation takes at least 15 minutes, and it takes longer if she prepares more than one-quarter of his spells.
These paragraphs specifically call out the wizard and cleric, but are those valid for other prepared casters? Can a magus leave open spell slots and prepare later? Similarly, can a druid? I'm not sure if this has been addressed officially at some point.
I'm fairly certain that these rules apply to everyone and they might better read Arcane casting and Divine casting.