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I have heard many times that a prepared caster can leave spell slots open to prepare later in the day as required, however Looking at the rules I'm wondering how useful that really is.
having a look at the CRB it does mention that this is possible.
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.
now the interesting part here is that it still required a mind fresh from rest, so even though it only takes 15 minutes to prepare the spell you still need to get some rest.
now the part that is confusing to me is how much rest is required for this looking earlier on this page.
Rest: To prepare his daily spells, a wizard must first
sleep for 8 hours. The wizard does not have to slumber
for every minute of the time, but he must refrain from
movement, combat, spellcasting, skill use, conversation,
or any other fairly demanding physical or mental task
during the rest period. If his rest is interrupted, each
interruption adds 1 hour to the total amount of time he
has to rest in order to clear his mind, and he must have
at least 1 hour of uninterrupted rest immediately prior to
preparing his spells. If the character does not need to sleep
for some reason, he still must have 8 hours of restful calm
before preparing any spells.
so what defines rest is clear, but how long is needed for these extra sessions? 8 hours? 1 hour?
Reading this I can't see it being less then 1 hour but it seems unclear if the full 8 hours is required or not. can anyone cast some light on this?
Its only the 15 minutes.
You're taking the comment out of its context.
"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. "
That is a reference to a caster being able to do exactly that, in the first paragraph of that section, where it says:
"If a wizard already has spells prepared (from the previous day) that he has not cast, she can abandon some or all of them to make room for new spells."
It is the specific difference between memorizing a spell right after you rest and memorizing later in the day. The long rest period ends in you being 'fresh from rest' and is when you prepare your fresh spell-slots for the day. At this time you can abandon old spells to free up their slots. After that determination of free spell slots, it really doesn't matter when you memorize spells as the mechanic is the same:
it takes the larger of 15 minutes or M*T where M is the number of spells (or levels) being memorized, and T is the time required per spell (or level). T = 60/S but this is the place where the system is vague - S could be spells as singular units, or total spell-levels.
Say my wizard has 3 first level spells, 2 second level spells and 1 third level spell a day. Taken the first way, that's six spells to memorize and 60/6 it takes 10 minutes per spell. So memorizing just my three first level spells would take half the hour, but memorizing the one third level spell would take 10 minutes (but actually take 15 due to the 15 minute minimum). Taken the second way its 3x1 + 2x2 + 1x3 or ten spell levels, 60/10 = 6 minutes per spell level to memorize, and the three first level spells would take 18 minutes, but so would the one third level spell.
I prefer doing it by the total number of spell levels since I think that's a more accurate rendering of "his daily capacity" than just number of spells. The above never comes into play if you just memorize all your spells right after resting - it simply takes an hour. Leaving slots open makes you more flexible, but to misquote "with great flexibility comes [the need for] great mathmaticality." :)
The above post having been said, its also another place where minutiae can get in the way of play. I know DMs who just round it off and say 1/4 of your spells - 15 minutes, up to half - half hour, etc. And leave it to the player to do their own rounding and book-keeping while they continue their narration of the story. Others I know obsessively track every round and minute you use.
I think most DMs fudge time-tracking mostly, to keep things moving.
