A 3 ml bottle is meant to last around twelve weeks at one application a night. When someone runs out in six, it is almost never because they used it twice a day. It is one of three habits, and all three are easy to drop.
1. Loading the brush instead of wiping it
The applicator picks up far more than a lash line needs. Draw it out of the bottle and wipe one side against the neck on the way out. What stays on the brush after that is enough for both eyes.
If the line you draw looks wet, or you can feel it pooling at the inner corner, there was too much on the brush. It is not doing anything extra there — it is running off.
2. Treating it like mascara
A serum goes on the skin at the base of the lashes, not up the lash hairs. One pass, root to root, in a single movement from the inner corner outward. Painting up through the lashes uses three or four times the product and puts it where it has nothing to do.
Think of a very thin eyeliner, drawn with something much softer than an eyeliner pen.
3. Applying it to a face that is not actually clean
Oil is the problem, not make-up in general. A cleansing balm or a micellar water leaves a film unless it is rinsed, and a serum applied on top of that film mostly sits on the film. You end up reapplying because the first pass did not feel like it did anything.
Cleanse, rinse, pat dry, wait a minute for the lash line to be properly dry, then apply.
A note on the applicator
Do not rinse the brush and do not share it with anyone. It goes back into the bottle after every use, so anything on it goes into the bottle too. If you drop it, replace the bottle rather than wiping the brush and carrying on.
Two minutes of arithmetic
3 ml, once a night, is roughly 0.035 ml a night for twelve weeks. That is a quantity you cannot really see. If you can see how much came out of the bottle, it was too much — and the bottle will be empty in half the time for no benefit.