How close to you does your Familiar have to be for you to be able to dismiss it into its pocket dimension?
Within 30', to match the area you can resummon it into? Within voice range? Within 100', which is its telepathic communication range? Within 120' if you use the Message cantrip to make the command? Within any distance on the same plane, if you have Voice of the Chain Master? Within any distance on the same plane, regardless? Within any distance on any plane, if you cast Sending to make the command? Anywhere in the multiverse? The Find Familiar spell does not say.