"At any moment."
Translation:En cualquier momento.
Ningún just means the absence of something. Think of it like a negative to algún. Cualquier means any as well, but it is easier to think if it as "whichever" or "whatever." Another way to remember cualquier is to break it down to "cual" (what or which) "quiere". (To want) All together, whichever/whatever one wants. It's my understanding that "En algún moment" could work too, which would be more literal, but "En cualquier momento" moreso means (At whichever moment one wants, or at whichever moment.) Hope this helps and isn't too confusing.