Why does it take so long for crystals to grow?

The time it takes for crystals to form varies with the mineral that is taking shape and the environment around it. In the case of halite, which is the mineral form of sodium chloride, table salt, crystals can take shape over a few years or even a months, depending on the conditions under which it is forming. But other crystal minerals take thousands or millions of years to form. There are a number of ways to determine the length of the periods of time over which a crystal forms, but there are big variations owing to the specific minerals that are crystallizing and the conditions under which those crystals form. The chemistry, as well as the physics (heat, pressure) all play a role; these are the variables in the equation.