Oh yea, by the way, the whole knife is vulnerable to scratching. while quenching steel does make it harder it doesn't make it resistant to scratches, as many of us can sadly attest to.
the reason you would quench a blade in the fashion you described is so that the knife will be tougher, more resistant to breaking under heavy use because the edge is fully hardened but the spine is maybe only spring hardened.
as far as draw time, it's recommended to draw some "simple" steels for no less than an hour at temp. other steels require two hour minimum soak twice, some three times.
If this seems complicated, it's not. if you keep with this and do your research, you will understand. every steel is treated differently, and many knife makers treat the same steel differently. while there are standard heat treating procedures for each steel, it's almost like a recipe for bread, every baker uses flower and yeast but in different quantities. the result is bread but ever bakers bread is different.
Do you know what kind of steel you're using? if so, let us know.