I don't really feel an increase in effects when I reach a certain dosage of say, 400mg (I think it is different for everyone as everyone metabolizes codeine differently, but I am not 100% certain). I usually use codeine as a synergistic catalyst to combine with another weak opioid or even kratom (when I can't get anything else), and rarely do I exceed 400mg because I feel like it's a waste. When I use it with another weaker opioid at 200mg-300mg of codeine and a comparable dose of the other opioid such that combined with one another, it could be equated to 500mg-600mg codeine if there was no ceiling, I always get the feeling (because I too think about the codeine ceiling whenever I use it) that taking more than 400mg codeine, for me, does not provide any further effects that make it worthwhile, no matter how I administer it and no matter my tolerance level. In fact, staggering doses seems to negate effects sometimes. But that's just me. I know codeine has a metabolic time limit, so when you take a dose, you have somewhere between 45 minutes and 1.5 hours before your liver closes off the proverbial valve for 2-3 hours (roughly), so administering more will not be converted into morphine during that time. If you keep taking it over time, that time limit gets extended (like lean drinkers who drink slowly, the time limit before your liver shuts off the valve keeps being delayed with each sip).
I've heard people talk about how they use 500mg to 1g codeine and they continue to get further effects - which leads me to believe they are not metabolizing the codeine into morphine at a sufficient rate or quantity to begin with (compared to others), and for them, administering those very high dosages would be like me taking 400mg. But that's purely conjecture and my guess at what it could be.
