Most carabiners are made of aluminium alloy. In theory, metal parts have an unlimited lifespan. However, in practice, material wear can become a factor since bolt hangers or rock are harder than aluminium. Prolonged use where the rope runs may also cause gouging on the inner surface of the carabiner, creating burrs and marks that could in turn damage the sheath of your rope. Larger diameter carabiners usually last longer as they have a larger rope-bearing surface. We recommend that you inspect your carabiners on a regular basis.
For more information about carabiners, draws and slings, see this article.