The wood will soak in the amount of stain that it can for however long you leave the stain on the wood, typically about 5-10 minutes before wiping it away. The piece may look like it lightens, but the excess stain is being wiped away. Another reason it may look like it lightens as it dries, is that the stain penetrates deeper leaving the top layer with less stain as it gets absorbed. A good rule of thumb is to keep reapplying until you achieve the color you want. This gives the wood the opportunity to absorb more stain. However, never let the stain dry on the surface without wiping away the excess because it will get gummy and sticky.