Your studies and talents in magic allow you to put more of your magical energies into spells you cast to extend their duration.

Used on its own this cantrip does nothing but this cantrip can be cast while casting anothing spell. If the other spell being cast is an action or greater time this cantrip takes a bonus action but if the other spell is a bonus action this cantrip takes an action. This spell cannot be used with spells that use a Reaction. The other spell being cast must have a non-permanent duration greater than Instantaneous such as 1 round, a minute, 1 hour, etc.

For the sake of simplicity we will call the other spell the 'Target Spell'.

When you use this cantrip you can sacrifice any number of spell slots of any level and can spend these levels to extend the Target Spell's duration. You must spend the same amount of levels as the Target Spell's level - at the level it is being cast - and each time the base duration of the Target Spell is added to it.

For example: you cast Unseen Servant which is a 1st level spell with a duration of 1 hour. While casting that spell you use this cantrip and sacrifice a 2nd Level spell slot and spend these. For each level spent, since the target spell is 1st level, you add 1 hour. Your Unseen Servant now has a duration of 3 hours. As another example let us say you cast Fly at level 5 so you and two friends can fly. You use this cantrip to extend the duration but since it is cast as a Level 5 with a base duration of 1 hour it will take 5 levels to increase the duration by an hour so you could sacrifice a 2nd level spell slot, a 3rd level spell slot, and a 5th level spell slot which is 10 spell levels which will let you extend the duration of your 5th level Fly spell by another 2 hours - you and your two friends can now fly around for 3 hours (providing you can maintain concentration for the full 3 hours since it is a concentration spell).