I'm considering adding a heated bed to my printer so that I can start using ABS.
But first some questions.

Approximately how many layers is the heated bed useful for?

Lets say you are doing a print that has 200 layers. Does the heated bed affect all 200 layers or is just the first few?

I suspect that it is just the first few - if so is there a way to turn off the heated bed after the first say 20 or 30 layers? I suspect not but I thought I would ask.

The heat of the heated bed is needed to keep the printed objects sticking to the building plate. If you stop heating before printing is finished, your objects will get loose and will move.