How long can I store the Dynacem (in a dry and cold place 0 to 20 degrees Centigrate) to have it still in good working condition?

You can store Dynacem in unopened bags 1 year from purchase day or longer.
After 1 year you feel no difference in field performance.
After 2 years it breaks the same rock but it needs 2 days (3 days for reinforced concrete) instead of 1 day.
After 3 years it needs 3 days (may not work with reinforced concrete).
Opened bag will decay faster, but you can vacuum pack it to keep fresh for another 1 year.
You can vacuum pack unopened bags to keep fresh for 3 years (and probably longer).