Megger is a generic term for mega-ohm meter, like Romex is commonly used for non-metalic sheathed cable. Megger is a mfg. It is used to test the dielectric quality of an insulated conductor. It can also be used to drive worms from damp earth and cause fish to jump out of water (like a crank telephone).
It is pretty simple to use. The wire is disconnected from all devices, isolated from any grounding system or device connected to the megger, and then a voltage is hankcranked into it. The better the insulation, the longer the wire (now a capacitor) will hold the potential voltage.
The voltage selections for a good megger are high enough to kill and can destroy any equipment accidently still connected to a circuit, so it is not a test for a novice.
Usually, only high current conductors, large service or supply conductors are megged, but some picky commercial or tele-communication companies require a meg test for even small and low voltage (120 volt) systems before they will entrust connection to their equipment.
For residential use, systems of 400 amps at 240 volts or less, meggers can be used to test questionable insulations but generally a simple continuity test will seperate the shorted wires from the usuable ones.
Sometimes a megger is used to Hi-pot large high voltage wires that have been in a state of disconnect or non-use for a period of time. Water exists in all raceway systems to some extent. Conductors in use will keep transient moisture dried out, but over time it can creep into systems disconnected for construction or maintenance. A skilled electrician or engineer might be employed to crank increasingly higher voltages into one of these at rest conducting systems to dry it out over a day or two before use to avoid damage to a damp leaky but otherwise useable system with a sudden high voltage connection. Note:this procedure is very rare and usually involves an engineering company even for very large sophisticated electrical contractorsl.
I suppose it could be used to dry out a wet residential system, but I've never heard of it. It would be cheaper and more reliable to just replace a residential wiring system exposed to that kind of moisture.
How long does it take. It takes about sixty seconds to crank a test plus ever how much time it takes an electrician to disconnect all devices to each wire tested. Multiple device circuits would require disconnecting each device and testing each segment of that circuit.
That should be about all you should ever need to know about meggers, except don't touch a recently megged wire end until it has been drained of potential. They can hold residual voltages for a while.
Hope that helps,