In Low.Power.Sleep sleep how time to increase sleep time?

My code:

else
{

Serial.println("I am in else");
String val = "Data from N1";
char radiopacket[500];
nodetx();
digitalWrite(LED_BUILTIN, LOW);
USBDevice.detach();
LowPower.sleep(1000);// here how may times loop should run for 20minutes??
USBDevice.attach();
digitalWrite(LED_BUILTIN, HIGH);
Serial.println("Awake");
return;
}

I want to increase sleep time for 20minutes please suggest what to add in this code, as I know I have to run this through for loop
2. Also want ot know maximum by default? Is it 2min? As I am getting maximum sleep as 2min, nit beyond that
3. Is there any calculation method then please suggest.

If you want comments on how to improve your code, you need to show the code to the forum.

The code you have provided, is not the real full code is it ?

Forum members are a bit short of Crystal balls at present.