| voidfoo |
Adding a limit seems odd given that many sustained spells make a point of how far one can be from the effect.
That is an interesting point I had not noticed before. Searching the books yielded spells like Dancing Lights and Project Image which mention this. Also there is Magic's Vessel with a range of touch, where it would be odd if you couldn't sustain outside that range.
Dancing Lights
Range 120 feet; Duration sustained
You create up to four floating lights, no two of which are more than 10 feet apart. Each sheds light like a torch. When you Sustain the Spell, you can move any number of lights up to 60 feet. Each light must remain within 120 feet of you and within 10 feet of all others, or it winks out.Project Image
Range 30 feet; Duration sustained up to 1 minute
You project an illusory image of yourself. You must stay within range of the image, and if at any point you can't see the image, the spell ends. Whenever you Cast a Spell other than one whose area is an emanation, you can cause the spell effect to originate from either yourself or the image. Because the image is an illusion, it can't benefit from spells, though visual manifestations of the spell appear. The image has the same AC and saves as you. If it is hit by an attack or fails a save, the spell ends.Magic's Vessel
Range touch; Targets 1 creature; Duration sustained up to 1 minute
A creature becomes a receptacle for pure magical energy sent by your deity. The target gains a +1 status bonus to saving throws. Each time you Cast a Spell from your spell slots, you automatically Sustain this Spell and grant its target resistance to damage from spells until the start of your next turn. This resistance is equal to the level of the spell you cast.