SRD:Control Shape Skill

Control Shape (Wis)
If you have contracted lycanthropy and are aware of your condition, you can learn Control Shape as a class skill. (If you are an afflicted lycanthrope not yet aware of your condition, you can attempt Control Shape checks untrained.) This skill determines whether you can control your shape. If you are a natural lycanthrope, you do not need this skill, since you have full control over your shape.

Involuntary Change
You must make a check at moonrise each night of the full moon to resist involuntarily assuming animal form. If you are injured, you must also check for an involuntary change after accumulating enough damage to reduce your hit points by one-quarter and again after each additional one-quarter lost.

On a failed check, you must remain in animal form until the next dawn, when you automatically return to your base form. If you are aware of your condition, you may make one attempt to return to humanoid form (see below), but if you fail, you remain in animal form until the next dawn.

Voluntary Change
In addition, if you are aware of your condition, you may attempt to use this skill voluntarily in order to change to animal form, assume hybrid form, or return to humanoid form, regardless of the state of the moon or whether you have been injured.

Action
Voluntary changes are standard actions.

Try Again
You can check to resist an involuntary change once each time a triggering event occurs.

You can retry voluntary changes to animal form or hybrid form as often as you like. However, on a failed check to return to humanoid form, you must remain in animal or hybrid form until the next dawn, when you automatically return to humanoid form.

Special
If you are an afflicted lycanthrope, you cannot attempt a voluntary change until you become aware of your condition (see Lycanthropy as an Affliction).