I pick an object about 7-10 feet away and see how long I can stare at it without my eyes reverting to something else. I'd recommend starting with 1 minute. The trick is to try and relax while you're doing it. And yes, you are allowed to blink, you just can't move your line of sight from that specific target.
I used to do the same thing, and would specifically focus on elements of license plates as I was driving to the pool hall to practice.
I have a theory about this.
From an evolutionary perspective, what we call "dead stroke", and other sports/games have various different names for, is a state of utter focus, where all extraneous data is ignored, and all important data is prioritized. Reflexes are heighetned. Muscle movement is extremely refined and accurate. You are extremely aware of all "important" inputs coming in. I believe this to be an evolutionary adaptation that helped our hunter-gatherer ancestors operate at ultimate efficiency, under certain conditions.
The longer it has been since a successful hunt, and the hungrier you are.. The more likely you are to be EXTREMELY focused, when you are both actively seeking prey, and when you find the prey. The brain interprets this as "I am hyperfocusing for a long period of time... Whatever I am doing, seems to be super important to my survival. Time to trigger all the brain/body mechanisms that will help me with whatever I am focusing on..."
As such... Doing the type of targeted focus exercises you do, is "training" the brain to go into dead stroke. It's not just an exercise to improve eye muscle endurance (though it does that too..).
What is interesting about this is.. Some form of "dead stroke" exists in almost every human pursuit that requires fine muscle control, or heightened brain function. I achieved it in poker, and it became a lot easier to read people's probable card ranges, because I paid close attention to EVERY hand, and what each player did, in what position, how much they bet based on likely card range, etc. It felt exactly like pool dead stroke. Reading players got to be super easy when they didn't properly randomize their betting/playing patterns. My wife was super entertained when I would accurately call an opponent's hand, and then manipulate the betting pattern to induce them to go all in when they were dead money.
Now, that being said.. My only experience with this type of heightened performance has been exclusively in games that required a heavy "eye-brain" cooperation... Pool and Poker. But I expect that it also applies in games/sports with less of a visual aspect (Rubik's Cube, for example). Rubik's Cubers doing speedcubing require ultra precise muscle control and brain participation, but the visual aspect in the moment is not as necessary, as the brain basically knows all motions in order to get from current state to 'solved". The visual aspect is simply not as important, and their speed is not greatly affected when blindfolded.