How much past the lock nuts does the axle extend is a "depends" answer.

First is whether the axle is a QR design or an axle nuts design. Obviously the nutted version needs enough axle extension to allow for the drop out thickness (and der hanger plate, claw) and that of the nut so the nut is fully threaded on when tightened. Generally I aim for the same amount of axle showing past the nuts on both sides. For a QR axle the axle ends shouldn't extend more then that of the drop out thickness, and a bit less is the typical. Remember that the QR conical springs take up a small amount of space between the axle end and the QR end caps, you don't want the QR to not fully bite into the drop out because the axle/springs stick out too far.

As mentioned if the der is attached with a "separate of the frame" hanger (these days often referred as a "claw") the effective drop out thickness will be more then that of just the frame. So a bit more axle extension on the drive side in these cases is allowable.

I aim for about the same amount of axle extension on both sides unless something suggests otherwise. Andy