How long does chocolate soothe a bad mood? Only 3 minutes according to a study by Macht & Mueller (2007) in the journal of Appetite. After that, the effect dissipates.
However, the mood elevating properties depends on how good the chocolate is. Believe it or not, all chocolate is not created equally. In their study, subjects received different types of chocolate after the experimenters induced a bad mood. The impact on a negative mood only worked with the "palatable" or "good" chocolate.
The researchers hypothesized that the positive effects were due to the immediate sensory pleasure (taste on the tongue) and emotional associations with chocolate. It's unlikely that the chocolate impacted neurotransmitters or chemicals in the brain. This would generally take up to an hour. "Emotional eaters" seemed to get the most impact from the chocolate.
Source : http://www.psychologytoday.com/