How long a set of ampoules lasts
A set is four sealed ampoules of ten millilitres each, plus one refillable roller head.
About thirty nights
At nightly use on a face, one ampoule lasts roughly a week, so a set of four runs about a month. Using it on arms and legs as well will go through it faster.
Why they are sealed separately
A sealed ampoule is not sitting open on a shelf for a month. You open one at a time and the other three stay as they were.
How the subscription is timed
Monthly, because that is roughly when a set runs out. It can be skipped or cancelled from the link in every email, and there is nothing to phone.