How fast in DnD terms is the word "immediately"?

For example, the spell Sending says:

You send a short message of twenty-five words or less to a creature
with which you are familiar. The creature hears the message in its
mind, recognizes you as the sender if it knows you, and can answer in
a like manner immediately. The spell enables creatures with
Intelligence scores of at least 1 to understand the meaning of your

Does this mean within one round/6 seconds? Or does it mean within the creature’s next action?