The honest answer is every two to three years for a horizontal deck surface and every four to six for vertical fences and railings. Decks take the worst of it — standing water, foot traffic, and full sun in summer — so they wear faster than anything else on the property. If water no longer beads on the surface after a rain, the finish is done and the wood is absorbing moisture.
Stain type changes the math. Transparent and lightly tinted penetrating oils show the most grain and are the easiest to recoat, but they wear fastest. Semi-transparent stains last longer and still show wood texture. Solid-color stains behave more like paint: the most protection and the longest life, but they can peel if applied over a surface that wasn't properly cleaned and dried.
Prep is where most do-it-yourself stain jobs go wrong. Wood has to be cleaned with the correct pressure (too high furs the grain), treated with a brightener to open the surface and neutralize the cleaner, and then allowed to dry fully — usually two or three dry days — before stain goes on. Staining damp wood traps moisture and leads to the blotchy, peeling finish everyone has seen.
Timing matters too. We stain in the summer window when there's a reliable dry stretch and mild temperatures; stain applied in direct hot sun flashes off before it can penetrate, and stain applied in October may never cure before the rain returns. Get on the calendar in spring and the deck will go into fall protected.