By definition, that time is always 1 or 2 seconds, and mostly depends on the preference of TTN.
(If your own code can somehow know if an ACK was received depends on the library you’re using. A good library will just re-send the message if no ACK is received, without bothering your own code with that, hence might not even make that information available to you…)
Note that TTN Console will also show the ACKs and retries.