When or if you become pregnant, when will your stomach harden, an when will you notice it?

Depends: Everyone notices their own bodies differently, but before 10 weeks, everything is hidden inside your pelvis and there is nothing to see. After that, you might start to notice visible changes.

12+ weeks or so: Typically the uterus as it enlarges, does not rise above the pelvic bone until 12 weeks or so. It is then when you can feel it and also starting to show the pregnancy "hump"