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d20srd.org says :

When you begin a spell that takes 1 round or longer to cast, you must continue the concentration from the current round to just before your turn in the next round (at least). If you lose concentration before the casting is complete, you lose the spell.

Which declares the need for 'concentration' to cast the spell. How then can a Character keep concentration on a spell for 10 minutes as with Arcane Eye or 24 hours(!) as with Awaken?

Particular topics to touch include, but are not limited to: food and water, standing for extended duration of time, keeping concentration, casting on a plane with 'timeless' attribute, and regaining spells for the next day (given the Awaken example).

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Going without food or drink for 24 hours is unpleasant, but it's not going to seriously injure you or anything. It's pretty much par for the course before many types of major surgery, for example. Going without rest for 24 hours is similarly well within the realm of things human beings can do without suffering any long term effects. I have personally done each of these things (though I have not ever casted a spell that required doing so :P). –  KRyan Apr 12 at 20:54
10 minutes is not that long, unless your character has a smartphone ;-) For many spells longer casting times seem to prevent combat use (Arcane Eye). Also, "concentration" doesn't mean "standing still", all verbal and somatic components might be employed throughout the whole casting time (Awaken can be imagined as a specific ritual with chanting, dancing etc.). –  burlap Apr 12 at 21:59

1 Answer 1

You Just Can

The rules have next to nothing to say about this. Concentration (and the Concentratinon rules for casting spells) lists conditions that can cause you to have to roll to keep concentration. If none of those are in effect and your DM doesn't rule that you have some kind of problem, you simply can. No rolls or checks are required simply because the spell takes a long time, the conditions that require a concentration check are listed there.

Food & Water

Food and water are not mentioned as requirements for concentration. They are mentioned under Starvation and Thirst:

Characters might find themselves without food or water and with no means to obtain them. In normal climates, Medium characters need at least a gallon of fluids and about a pound of decent food per day to avoid starvation. (Small characters need half as much.) In very hot climates, characters need two or three times as much water to avoid dehydration.

A character can go without water for 1 day plus a number of hours equal to his Constitution score. After this time, the character must make a Constitution check each hour (DC 10, +1 for each previous check) or take 1d6 points of nonlethal damage.

A character can go without food for 3 days, in growing discomfort. After this time, the character must make a Constitution check each day (DC 10, +1 for each previous check) or take 1d6 points of nonlethal damage.

Characters who have taken nonlethal damage from lack of food or water are fatigued. Nonlethal damage from thirst or starvation cannot be recovered until the character gets food or water, as needed—not even magic that restores hit points heals this damage.

Unless you have a casting time of 72 hours or are already starving, food is basically never an issue. If you have a Constitution of >= 1 (aka: are alive), water isn't an issue if you remember to drink before starting, even for a 24 hour casting time. Note that this is Constitution score, not modifier. Even a Constitution 2 character can go 26 hours without water before suffering damage.

If you did run into the starvation rules while casting, the rules would trigger a Concentration check as per the casting rules:

If while trying to cast a spell you take damage, you must make a Concentration check (DC 10 + points of damage taken + the level of the spell you’re casting). If you fail the check, you lose the spell without effect. The interrupting event strikes during spellcasting if it comes between when you start and when you complete a spell (for a spell with a casting time of 1 full round or more) or if it comes in response to your casting the spell (such as an attack of opportunity provoked by the spell or a contingent attack, such as a readied action).

If you are taking continuous damage half the damage is considered to take place while you are casting a spell. You must make a Concentration check (DC 10 + ½ the damage that the continuous source last dealt + the level of the spell you’re casting). If the last damage dealt was the last damage that the effect could deal then the damage is over, and it does not distract you.

Repeated damage does not count as continuous damage.

Nothing in the rules says you have to stand to cast these spells, and nothing in the rules says you can't stand for 24 hours to cast them.

Regaining Spells

Arcane Spellcasters who cast a 24 hour spell would need to sleep after casting it, then they can regain spells normally.

Divine Spellcasters would have to pray immediately, as per the preparation rules (the first opportunity is after you finish casting, as Divine Casters don't need rest to prepare spells):

A divine spellcaster chooses and prepares spells ahead of time, just as a wizard does. However, a divine spellcaster does not require a period of rest to prepare spells. Instead, the character chooses a particular part of the day to pray and receive spells. The time is usually associated with some daily event. If some event prevents a character from praying at the proper time, he must do so as soon as possible. If the character does not stop to pray for spells at the first opportunity, he must wait until the next day to prepare spells.

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