Green stuff hardens to a firmness akin to the plastic used for most action figures, not quite rigid.
If you want a harder finished product try using 'brown stuff', another form of Kneadatite that dries hard and is sandable. If you go over to Reaper and look at the greens you will find that some sculptors use a mix of the two, or make the bodies with the green stuff and the weapons with brown stuff.
You can also make a 'curing box' a closeable box with a light bulb to warm it up inside. In winter a radiator works just as well.
The Auld Grump