With my allergist at least, the dosage is determined on how much of the allergen you tolerate in the shot.
You start out with a certain standard amount of allergen. If you don't get a reaction (usually a wheal), then they raise the concentration, probably tenfold, for the next batch, usually after a month of treatment. They keep going until you react, then drop the dose down a notch.
Often the amount of the dose is raised during the month, going up a week at a time. Starting out at .3 cc, going up to .5 cc, then .75 cc to a whole cc.
Some of us react to the very first shot. I did. My dosage had to be reduced by a factor of 10 twice to get down to a dose I tolerated. Then I was titrated up as far as possible.
You may then stay on this dose for years. If it stops working your allergist may have to add an antihistamine or reevaluate for an additional substance. I became allergic to grass after a few years and had that added.