Wood fences turn gray for a reason: the sun breaks down the surface fibers, and rain washes them away. Once that starts, the boards absorb water, swell, crack, and warp. A coat of stain or paint is what keeps that cycle from starting.
In our climate, a semi-transparent stain typically lasts two to three years before it needs a fresh coat. A solid-color stain lasts three to five years. Exterior paint over a well-primed fence can go five years or more, but it shows wear differently, peeling instead of fading. Fences that get afternoon sun or sprinkler overspray are on the short end of every range.
The easiest test: sprinkle water on the fence. If it beads up, the finish is still working. If it soaks in and darkens the wood right away, it's time to recoat.
Good fence work starts with a pressure wash to remove dirt, mildew, and loose gray fibers, followed by a full day or two of drying. Loose boards get secured, then the finish is sprayed and back-brushed so it works into the grain instead of sitting on the surface. Done that way, a fence stays protected and looks intentional instead of forgotten.