Answer:
If the sustainability in question is environmental sustainability, chocolate is both a power- and labor intensive industry. This means that on top of the energy costs of running an industrialized cocoa plantation, chocolate often travels great distances from its country of origin.
The highest quality of chocolate often comes from Latin American cocoa plantations wherefrom it's flown to Europe for processing and packaging, wherefrom it's distributed throughout the world. Therefore a lot of fossil fuels are used in the production process, chiefly in the transport of chocolate.
If you meant how well chocolate keeps, I'd say about 3-4 weeks in the fridge.