By the book
20', no farther, as already posted. But a good DM will know that some things are easier to spot in darkness, like how a cat's eyes will catch and reflect incoming light. A cat looking at you will show its eyes at distances greater than the technical limit of the torchlight, etc.
Additionally, at higher levels characters may be able to make insane spot DCs like 35-40+. The rules (at present) are gray on the subject of extreme DCs because WoTC plans to publish a book on the subject of Epic Level gaming at some point.
You'll probably want to look into these new rules as soon as they are available.