Pay attention to the position of your track nuts and see if they move in the dropouts at all. If not, your chain is being stretched out. If your chain is too tight it will wear out very fast. Dirt and grit on the chain add to this as well. If your chain line is good the chain can be a little loose without any negative effect. You should not hear any clicking or crunching.

As Filtersweep stated, the chain isn't actually "stretching", the bushings are being worn away. The visual effect is the same though, and having the chain too tight creates uneccessary friction and wear.