Current drain of typical electret mic?

very small for this type. usually hundreds of hours

yes. if there is no off switch take the battery out when not in use.

I do not know of any that use a battery inside; all of those i have seen get power from the instrument (like the stereo mike input on a computer - one side is sound and the other side is the power). In any case,the power is in series from source to mike - meaning if the mike is disconnected, no current would flow.

Here is one with a battery inside:

Some amplifiers have internal links to provide power for electret microphones, over the microphone lead. Many do not.

electrets should not draw any power when not plugged in

Phantom power is a 48v signal from your desk

you are perhaps thinking of computer or camcorder type mics which get operating voltage from the device they are plugged into. there are many types that do require a battery or some other voltage supply.

You are referring to mics with Plug-In-Power or those with phantom-power. Other mics have the battery inside.

the batteries in a typical electret mike usually last several years. the draw is so low it is the shelf life of the batteries that matters. they self-discharge internally with time.

-bob