20 seconds is pretty long. Your opponent is up 2 workers on you, so assuming perfect micro from both sides, the defender can use one worker to continue to mine

20 seconds is pretty long. Your opponent is up 2 workers on you, so assuming perfect micro from both sides, the defender can use one worker to continue to mine minerals and pump out more workers, while microing with the other 11 vs the attacker's 10 workers.