There is no such thing - you send ON command and that is then translated to a “button push(ed)” packet that is send to Nikobus instalation. So one ON command generates one “button pushed” packet send to Nikobus installation. There is no “ON time”.
As explained before, Nikobus hardware push button, when pushed (and hold) will generate the same “button push(ed)” packet every ~50ms so IMHO you should mimic that behavior, so have a loop with ~8 loops, sending ON command and wait for ~50ms between each send command. Please also note push button will only react to ON command, ignoring other commands so expire might not get you results you are after …