You can voluntarily "forget" a spell at any time, to open up a slot so that you can learn a new spell. Note that using up all of a spell's charges does remove the spell from memory, but of course you have to use END to do so. The way in which you change your spells should normally be defined when you build you character, and will be dictated to a large extent by the sort of wizard you are. A nasty old necromancer might have to bathe in the blood of newly-killed two-day-old infants to recharge his magical energies so that he can undertake the arduous task of learning a new spell. A shaman might have to spend several days out of his gourd "purifying" himself. Mister Fluffy the Magic Man may psyche himself into the task by thinking a thousand Happy Thoughts. It's really up to you, but as a rough guide to the minimum time-frames I will allow, you can use the following: Removing a spell from mind takes one phase per active point in the spell; in an emergency the process can be speeded up to one segment per active point, but this will penalize the skill-roll for the spell filling the vacated slot by one per 10 active points in the deleted spell. (Spells can sometimes be instantaneously removed from memory by taking an Impairing blow to the head.... this method is not recommended for day-to-day use).

Learning a new spell is a slower process, and cannot be speeded up at all. It will take you one minute per active point to place a new spell in mind, but once there it remains available until voluntarily removed. (The time constraints on changing the spells in a Variable Magic Pool are slightly different.) If you are disturbed during the learning process you must make a Magic Skill roll @ -1 per 10 active points in the spell, or else start again from scratch.