Berry green lentils have a two year "best before" date and are sold the same year they are harvested. They are the only dried legumes for which this is the case. You can store them in their bag whether it is made out of hessian or plastic, or in a plastic container for example. The package and the product itself should not be stored in a damp place.
Tip: Store in a dark place; lentils are not fond of the light, otherwise they will change colour and redden.