I have a MIDI keyboard connected to the Apple GarageBand DAW (digital audio workstation) app. Is there an effect setting where I can have a note repeat at a set interval (or tempo) when I hold down a note on the keyboard? Kind of like an infinite echo effect that doesn't fade out?
You would need to find a (commercial paid or free) third-party AU (Audio Unit) plug-in such as a synthesizer instrument that includes a MIDI arpeggiator to use within GarageBand. This is a virtual device that will repeat the notes in a chord that you hold down on the keyboard in a pre-defined sequenced arpeggio pattern which is synced to the tempo of the GarageBand track to which you are recording a part. As the name would imply, arpeggiators are chiefly for working with chord patterns, but I think you could find one that would work on a single note as well.
You might also have success with an AU plugin that provides an audio looper, which would accomplish a similar effect with audio rather than with MIDI.
Do a Google search on "virtual instrument arpeggiator" or "virtual instrument looper" and see if you can find a suitable plug-in that is GarageBand-compatible.