Continuatum

The greatest mages of the realm are able to freely control the movement of their magic, and some have mastered the ability to transform their concentration into a self-sufficient, persistent effect. While concentrating on a spell, you may extend the duration of the spell longer than your concentration would normally allow. This spell has no effect on instantaneous spells; it only affects spells that require concentration. Additionally, this spell can only be cast on spells of 7th level or lower (with no effect on cantrips). For spells lower than 4th level, casting time is equal to a number of days equal to double the spell's level. For 5th level, the duration is one month. For 6th level, the duration is 3 months. For 7th level, the duration is 1 year. While concentration in this manner, you don't have to devote all of your attention to the spell, but you cannot cast any spells of any type. If you do, the concentration is broken and Continuatum's duration ends.

If the casting time is endured to completion, Continuatum and the spell in question exist indefinitely until dispelled, and neither spell requires concentration any longer. The effects of the spell persist as long as the spell is active, requiring no verbal, somatic, and material components for the spell to continue. If you attempt to continue multiple instances of the same spell, Continuatum instantly fails. You may only have as many instances of Continuatum active at the same time equal to your spell casting abillity modifier. If Continuatum is countered (via counterspell or by any similar effect) at any point of its duration, or dispelled via dispel magic or any similar effect, the spell ends. If you choose to end the spell, both Continuatum and its effects end immediately no matter the distance from the original casting location. If cast via a wish spell, the duration of the spell is still required. If you die, all instances of Continuatum end.

If cast on a spell that summons a being from another plane (such as conjure celestial), the being may become permanently displaced from their home plane. If this occurs, your control over them is severed (if you had control over them).  If cast on a spell that allows you to control a being's actions (such as dominate person), your mind could become permanently affixed to the being. Make a DC 25 Wisdom saving throw; on a failed save, your concentration is permanently bound to the being, making it impossible to concentrate on any spell until you release your mind from the being. If cast on a spell that conjures a separate entity that you may control that is not a creature (such as flaming sphere), the spell could become more unruly as it continues. Whenever the effects of the entity are used past its normal duration (such as attacking with the flaming sphere), make an Intelligence (Arcana) check. If the check is less than 16 + the spell's level, the effects of the spell fail for that given use. If a creature under the effects of a Continuatum spell dies, the spell ends on that creature. If the spell changes the form of an entity or creature into something else (such as polymorph), the memories of the transformed entity or creature diminish as time moves on. While in this other form, the entity or creature will lose all of its memories of its former form over the course of 1d4 months + the spell's level. 

The GM has great latitude in ruling what occurs; the effects and consequences of using Continuatum are determined by the DM.