How do I reheat a casserole in the microwave?
There is no single way to answer this because:


Microwaves vary widely in power
The starting temperature of the casserole may vary
Casseroles vary in shape, size, thickness, and composition


and all of these factors will affect the time.

It is far from ideal to heat a full sized casserole in the microwave in any case; you would be better off using a conventional oven, as it will not have problems with hotspots.  Microwaves do much better with good that can be stirred to even out the heating, and this would be at the very least, aesthetically displeasing with most casseroles.

I would recommend starting with about 50% power (for a 1000+ watt microwave, higher for models with less power) to allow some time for the heat to conduct through the casserole.  Check regularly with an instant read thermometer until reads about 150 F at several spots.  

It is likely to be unevenly reheated, since you are not stirring.  This is the downside of using a microwave.  Allowing it to stand for 10 minutes or so before service will help even it out.

The other downside is that microwaving to reheat will do nothing good for crispiness, browning, or texture, but that is not unique to casseroles.