If you submerged a hand in liquid nitrogen for more than a second or two, you'd be getting some serious frostbite, and probably have to have some fingers removed. To totally freeze the hand, though, like what most people think of when they do the science experiments where they use a banana as a hammer, or shatter objects that are frozen solid, would take between 15 and 30 minutes. Remember that things like a flower or a banana are small objects, without any blood flow carrying heat to the affected areas, etc., and not a metahuman being.
Freeze bullets/guns, if they could get it to work would probably work along the lines of Stick-n-Shock ammo or flamethrowers, but with Cold damage instead of electric or fire.