The mass of a substance deposited or liberated at an electrode is directly proportional to the quantity of electric charge passed through the electrolyte.
Positive ions drift to the cathode, negative ions to the anode — deposit mass tracks the charge passed.
m = Z × I × t
m
mass deposited (kilograms)
Z
electrochemical equivalent of the substance (kg per coulomb)
I
current (amperes)
t
time (seconds)
In plain English: Every coulomb of charge escorts a fixed number of ions to the electrode, so twice the current or twice the time deposits exactly twice the metal.
Where you'll meet it: Electroplating and anodising calculations, copper plating of PCB through-holes, and the reason battery capacity is rated in ampere-hours — charge is chemistry.