How long does a dry cell battery last?

Hint: A substance which produces an electrically conducting solution on dissolving in a polar solvent like water is known as an electrolyte. The electrolyte on dissolving dissociates into its respective cations and ions which move uniformly in the solution and hence are responsible for electrical conductivity of the solution.

Complete answer:
A dry cell is an electrochemical cell which consists of low moisture immobilized electrolytes which are restricted from floating because they are present in a form of paste. It is the most common cell which is generally used for home and portable electronic devices. It works on the principle of conversion of chemical energy into electrical energy. There are many types of dry cell but the most standard example of dry cell is as follows:
Zinc carbon cell, sometimes also referred as dry leclanche cell. It is composed of a zinc another and a carbon cathode which are separated by ammonium chloride solution along with manganese dioxide which acts as an electrolyte for the reaction process.
The half-cell reactions for the process are as follows:
Reduction half-cell reaction:
 $ 2NH_4^ + + 2Mn{O_2} \to M{n_2}{O_3} + 2N{H_3} + {H_2}O $
Oxidation half-cell reaction:
 $ Zn \to Z{n^{2 + }} + 2e $
Overall cell reaction:
 $ Zn(s) + 2Mn{O_2}(s) + 2NH_4^ + (aq) \to M{n_2}{O_3}(s) + Zn(N{H_3})_2^{2 + }(aq) $
Dry cell batteries usually have a long life and can last ranging from 8 to 12 years and their service life is in range of 3 to 10 years.

Note:
It is important to note that a dry cell can further be divided into two types depending on the condition that it is rechargeable or not. If the dry cell is rechargeable then it is known as a secondary cell whereas if the dry cell is not rechargeable, it has a limited shelf life, then the cell is referred to as the primary cell.