Alternatively: just calculate it. The air time is also shown in TTN Console.
Next, for LoRaWAN Class A devices, the optional response will be after either a delay of exactly RX1 or RX2, hence after 1 or 2 seconds for a regular downlink…
Note that TTN will not send a response for every transmission. (LoRaWAN is not some TCP/IP; only downlinks, including OTAA Join Accepts and ACKs for confirmed uplinks, will yield responses. Most transmissions are just “fire and forget”, hoping that one or more gateways will pick it up and forward it to TTN.)