Why can substances remain electrically charged only for a temporary period? How can we determine the duration for which they remain charged? We know that a glass rod rubbed with silk loses its charge after sometime. Why is that? And what are the factors that determine how long the glass will remain charged?
 A: Opposite charges attract.   This means that a charged object, in the presence of any other matter, will pull at any charge that would neutralize the object.   It
will also repel any of its (excess) charge that somehow breaks free.
So, a charged object is unstable until it exchanges charges with its surroundings
in such a way as to become electrically neutral.
Discharge rate is hard to predict, though.  One contributor is humidity (water is
an ionizing solvent, water molecules sometimes spontaneously disassociate into
H3O+ and OH- ions), and another is ionizing radiation.   High temperature
also can cause ionization of otherwise stable molecules.
A: It is much harder to do electrostatic experiments in damp conditions.
The reason is that the materials which you would normally think of as good insulators become conductors.
It is not the bulk resistivity of the materials which changes, rather it is the surface resistivity which changes due to the layer of moisture on the surface because water is a relatively good conductor of electricity.
So there can be a conducting path from the object down to the ground along the surface of the object.
If the insulator is "dirty" there may also be other surface containments which are relatively good conductors
When it is damp the apparatus used in an electrostatic experiment is often dried using a fan heater to remove the surface layer of moisture.
To do your experiment you need to charge your object and place at a fixed position close to the cap of a gold leaf electroscope which has been recently dried and note the deflection of the electroscope as a function of time.
You can think of the electroscope as a voltmeter with a very high resistance and it can be calibrated by applying a series of voltage from a calibrated high voltage supply and noting the deflections.
The scale on the electroscope will be non linear.  
From your experiment you know how the electroscope voltage reading varies with time.
The electroscope shows a reading because of the charges induced on it by your charged object which depends on the number of charges on the object.
The smaller the charge on the charged object the smaller the voltage reading on the electroscope.  
The air can also be a conductor as there are always mobile charged particles in the air due to radioactive sources both natural and artificial (background radiation).
Objects which have a small radius of curvature can cause the air to be ionised and it could be that dust particles on you object or a sharp edge will cause a loss of charge.
