Personally I think it is absolutely inadequate - and not what Epicurus meant - to try to say something like "The limit of pleasure is met when pain is absent and so therefore once you obtain painlessness for a moment if does you no good to live a longer time."
Your comment "for a moment" is off the mark. It's not experiencing "the limit of pleasure" "for a moment" then going about your day. It's experiencing the limit of pleasure as part of your whole life, you experience life with this pleasure filling your mind and body. That's why ataraxia and aponia are important components of an Epicurean life. Once you are experiencing full pleasure without mental troubles or bodily pain, it doesn't matter if it lasts a moment and you die or you live 100 years then die or live an infinite number of years and die. He says, in this state at the limit of pleasure, "the mind does not flee from joy." There are innumerable ways to vary the pleasure, but you can't increase it once the limit has been reached. That's why - "reasoning it out" - a moment or infinity can conceivably contain the same amount of pleasure. Now, is this achievable for any being other than a god? Epicurus seems to think so because we are told that if we do, we live as gods among mortals.