Let your stone be the gauge. Just put some water on it; if it beads up, the sealer's still good, but if the water soaks in, your counter is due for a dose. Use only water-based, impregnating sealers, such as Sealer's Choice Gold from Aqua Mix. Unlike surface-type sealers, which can affect a stone's color, impregnators penetrate without altering appearance. Apply the sealer with a paint pad, let stand for 5 minutes, then wipe off the excess with a clean rag.