The difficulty adjustment only happens every 2016 blocks (~2 weeks), so the higher yields wouldn't happen unless they're down for an extended period of time.
That statement was to show why there is room to increase hash rate and not how you'd defend against a 51% attack. Most of the miners in the US are not mining for a significant portion of the day.
It depends on how long ago the previous retarget is and the current hashrate. The retarget is based on blocks, so if the retarget was 1008 blocks ago, but hashrate dropped by 50%, then it'll take 2 weeks until the next retarget.
