For all those printing right now

what if you wanted to to change to a different medium say from ABS to PLA how much material is wasted in the exchange I would think that there is material in the extruder at that point

So do you just turn it on to extrude the last bits and then put new filament in and print more until it comes out pure

I guess you can ask how much is in limbo at any given time stuff that has been melted but not extruded

 

Changing between materials doesn't result in too much waste because you can pull the 'old' filament out and leave only a small amount behind in the nozzle. I use a small allen wrench to push out as much residual material as I can before making a change.
PLA oozes out pretty cleanly and quickly at ABS processing temps.
Switching from ABS to PLA might take a little longer, but I've never really noticed a significant amount of time required.

Changing between colors of the same material type on the fly (z-axis changes) results in a little more waste so that you can get a clean color after the change. Maybe a minute of extrusion? I've never actually measured the waste, but its probably less than a meter. I've actually never done a z-axis pla change, but its probably less than with ABS due to the oozing.