When is equilibrium reached in diffusion?

Diffusion is the spontaneous movement of particles from an area of high concentration to an area of low concentration.

There are two types of diffusion: (a) a fluid through a fluid and (b) diffusion through a membrane.

Diffusion through a fluid

If you introduce a drop of purple dye into a beaker of water, the dye moves throughout the solution.

(From oerpub.github.io)

If the molecules are small enough to cross the cell membrane, they will tend to move toward the area where they are less concentrated.

Molecules can move across the cell membrane in both directions, but more move in from the side with higher concentration.

Eventually, the concentration of the molecules on each side of the cell membrane will be the same and particles will be going back and forth through the membrane at the same rate.

At that point, equilibrium is reached.