On all the time.

When you expect the brain to focus for hours on a stretch, it simply gives up. It will not put in the energy needed to do something tangible and make progress. Instead, it will distract us with stimuli whether it's email, social media or anything else.

To get deep work done, we need to create the conditions for it to happen. One of the conditions is to focus for not longer than 90 minutes at a time with a visible timer in front of you. You can do less but more is mostly not feasible.

Once you get into a habit of putting in 2-3 deep work sessions of 90 minutes each with a 30-minute no-screen break in the middle, you will see your productivity rise significantly and you will be able to avoid the "on all the time" hustle mindset.