In chemical reactions something has to be made. Chemical reactions are hard to reverse. Physical reactions tend to be changing state or dissolving things. Nothing is made in a physical reaction and they are easily reversed.
Reaction time is the time form the reaction starting to it ending. We know when it stops because the magnesium will completely disappear.
Magnesium + Hydrochloric acid ------------> Magnesium chloride + water
Mg + 2HCl -----------> MgCl2 + H2O