Clerical spells, including the druidic, are bestowed by the gods, so that the cleric need but pray for a few hours and the desired verbal and somatic spell components will be placed properly in his or her mind. First, second, third, and even fourth level spells are granted to the cleric through meditation and devout prayer. This spell giving is accomplished by the lesser servants of the cleric’s deity. Fifth, sixth, and seventh level spells can be given to the cleric ONLY by the cleric’s deity directly, not through some intermediary source. Note that the cleric might well be judged by his or her deity at such time, as the cleric must supplicate the deity for the granting of these spells. While the deity may grant such spells full willingly, a deed, or sacrifice, atonement or abasement may be required. The deity might also ignore a specific spell request and give the cleric some other spell (or none at all). The Dungeon Master will handle this considering a cleric’s alignment and faithfulness to it and his or her deity. Note that some cleric spells (and all druid spells) also require material ingredients in order for the desired effect to take place. Such components must be supplied by the cleric (or druid), as material is not bestowed.
Magical spells, those of the magic-user and illusionist, are not bestowed by any supernatural force. Rather, the magic-user (or illusionist) must memorize each spell, verbal and somatic components, and supply himself or herself with any required materials as well. Such memorization requires the character to consult his or her spell books in order to impress the potent, mystical spell formulae upon the mind. Additional items for the material component must then be acquired, if necessary.
Spells of any sort must therefore be selected prior to setting out on an adventure, for memorization requires considerable time.
As a rule of thumb, the DM should allow 15 minutes of game time to be spent in study/prayer and meditation in order to memorize/acquire the ability to cast one spell level, i.e. One 1st level spell requires 15 minutes or one 2nd level spell requires 30 minutes. Such activity requires a mind rested by a good sleep and nourished by the body.
Once cast, a spell is totally forgotten. Gone. The mystical symbols impressed upon the brain carry power, and speaking the spell discharges this power, draining all memory of the spell used. This does not preclude multiple memorization of the same spell, but it does preclude multiple use of a single spell memorized but once. When a spell caster shoots his or her spell-bolt, so to speak, it is gone.
Spell recovery, whether cleric/druid or magic-user/illusionist, requires about the same period of time. In order to pray and meditate for a new spell to replace one used, or in order to study and memorize such a spell, it is necessary that the spell-user rest and revitalize his or her mental faculties. Whether one or more spells are to be regained, the minimum time required for complete rest (usually sleep) is that required for the highest spell to be recovered. Minimum rest periods are shown below:
Thus, if a cleric or magic-user needs only memorize first or second level spells, he or she need only sleep for 4 hours and will then be able to memorize or regain as many such spells as he or she is normally entitled to. On the other hand, if the character in question also wished to include a seventh level spell, rest time would be 10 full hours, even though but a single seventh level spell were to be thus regained, while half a dozen second level spells were also to be regained.