I am on my first 30 ml bottle of liquid C. I have been using it over 6 months and have around half the bottle still left. I agree that the stuff settles out pretty quickly, but you have to remember that where you place the tip of the syringe is very important when you draw your dose. Assuming there is significant settling that occurs before you have a chance to draw the dose, then placing the syringe tip at the bottom of the bottle gives you a stronger dose and dilutes the remainder of the bottle. Conversely, drawing from the liquid surface gives a weaker sample and concentrates the bottle.
Drawing from the middle, or pulling the syringe from bottom to to top to try to draw from all levels, are techniques to keep in mind. Most important is probably just moving quickly to get your draw done after shaking. I haven’t really noticed a variation in the effect from dose to dose.