I just checked wikipedia for the chemical reactions you are talking about and suprisingly it's pretty accurate.

http://en.wikipedia.org/wiki/Bleach_(chemical)

You are dealing with the reactions of NaClO (aq) (sodium hypochlorite (aqueous)) + H2O (l) (if I have to explain this one, don't bother...). The NaClO when mixed in H2O is evaporating off at a much faster rate than the H2O. That's why the bleach concentration lessens from the mixture.

If you want the evaporation to happen faster, increase the surface amount exposed to air, get the mixture hotter, and increase the flow of air over the surface of the mixture.