According to the manufacturers instructions.
We do not know what you mean by "PFD", please do not use abbreviations. But the answer would be when the instructions for the "PFD" say it needs to be replaced.
You probably could fill it, but not to pressures needed for paintball gun operation, CO2 tanks should not be filled with any gas or liquid besides CO2. edit: you could use Argon in a HPA tank but not a CO2 tank
Whatever amount of pressure was used to compress the CO2 originally, can be reached (or nearly reached) when it is released into an air cylinder.
Normally it is done by weight. The empty cylinder weight is known and the weight with CO2 is taken and the CO2 weight is found.
You simply screw the CO2 into the back of the gun. Unless your CO2 should go elsewhere which is different to each gun.
It depends on the model, but most only need one.
disconnect the attached wire and unscrew the sensor. there is a gel that should be applied to the sensor before you replace it.
Indefinite. Usually they are tested & certified annually.
when its empty
I want to replace the co2 sensor in my crv myself is it something I can do?
The 2 should be written as subscript... CO2