If Eating Only 1 Big Meal A Day With Lots of Protein, How Long Before My Muscles Are In Danger From Catabolism And I Have To Eat Again?
If considering an “eat once a day” habit suits you, for whatever reason (say as a strategy to incorporate intermittent fasting), do it. Much of the answer will depend on the quantity, de facto processing qualities and macros manifest in the composition of the meal.
For instance, a meal featuring as its centrepiece a big, nicely marbled, grass-fed fillet steak, cooked medium rare with all the accoutrements may take between 8–12 hours to digest. On the other hand, a drink mix incorporating whey hydrolysate, moderate carbohydrates and only a few grams of fat might love you and leave you within a few hours.
It could take at least this time to complete digestion, and probably several hours longer, before you begin to hear the reverberations of muscles screaming for food and threatening auto-canabalism and urging you to start thinking about your next feeding.
Metabolic rate, of course will have direct bearing on this matter. It is also apparent that your age, fitness level and activity type will have impact on relevant events. Furthermore, whether or not you decide to try to sandwich a heavy resistance training session, swim across the Channel or a attempt a marathon run in between meals will create and modify additional demands.
NOTE* Perhaps a little self observation and experimentation may be called for to determine what the sweet spot for you might be. A simple pencil and notebook or diary app could be an invaluable set of tools and worth your while here. Also, a high dose of BCAAs (especially Leucine) and Creatine Monohydrate will be a good boost to muscle protein synthesis, if you are not planning to eat a couple of hours subsequent to a resistance training session or other high intensity bout of exercise.
But, if not, then as a rule of thumb:
For a heavy protein and fat laden meal 12–15 hours;
For a lighter low fat protein meal (your average protein shake) with medium to high carbs 2–4 hours.
…and various rates for various foodstuffs, in between.
Lots of first world athletes, bodybuilders and armchair experts will uncategorically state that you must not eat more than 30g. of protein (because that is the maximum amount that the body can absorb in this period of time) every 3 hours. But it is evident that it is probably the total amount of protein within a certain period, possibly 24 hours, that is really the key.
[…} it was demonstrated that a positive linear relationship existed between incrementally higher protein intakes (from ~6 g to 92 g) and whole body NPB that led the authors to conclude that no practical upper limit exists regarding the amount of protein that could maximize muscle anabolism 
As long as they have the opportunity and resources to pursue this strategy, it certainly won’t hurt anybody to follow the “Every 3” advice. Even if the truth of it, for various reasons, is in doubt.
Others who choose not to follow this familiar advice but feel a gun pointed at their heads shouldn’t feel at a disadvantage or alarmed that they will shrink down to the size of gnats, if they don’t go along.
In nature little creatures often eat less but more frequently than larger creatures, who tend to eat more, less frequently. My own instinct is that it would have been a somewhat unwieldy idea, if not a downright sinister mind-boggle, for evolution to have designed this sort of hummingbird-like level of frequency demand into its official evolutionary strategy for humans.
But I imagine there will be others much more qualified than myself who would be willing to argue the point.
So, as always