Being an exponential function, the saturation curve only approaches, but does not reach, the asymptote (complete saturation). Therefore, I suppose if you want to get really technical, the correct answer would be that saturation only occurs at the limit as exposure time approaches infinity. Therefore, we are never truly saturated or truly at equilibrium. That's one correct answer, although it's entirely useless for planning dives.
Again being hypertechnical, gas molecules continously transit between cellular boundaries (e.g. blood and muscle tissues) due to random molecular motion, and "saturation" or "equilibrium" is the theoretical point at which approximately the same "quantity" of gas molecules diffuse into and out of cells and with approximately the same degree of frequency. So one can also accurately say that ongassing and offgassing occur constantly due to random molecular motion. This is also a technically correct, but practically useless, answer.
Bob provided the practical and useful answer.
Again being hypertechnical, gas molecules continously transit between cellular boundaries (e.g. blood and muscle tissues) due to random molecular motion, and "saturation" or "equilibrium" is the theoretical point at which approximately the same "quantity" of gas molecules diffuse into and out of cells and with approximately the same degree of frequency. So one can also accurately say that ongassing and offgassing occur constantly due to random molecular motion. This is also a technically correct, but practically useless, answer.
Bob provided the practical and useful answer.