Credit remains available for an unlimited period, according to the FAQ answer to the question: "Wie lange ist das aufgeladene Guthaben für die Nutzung verfügbar?" - How long does the top up credit last?
https://www.netzclub.net/faq.5.html
To keep your free data allowance active, you need to check and react to the netzclub SMS adverts. You will receive these free, wherever you are in the world. You can also check for the SMS adverts online by looking at your netzclub account history (here you can see a complete list of sent adverts and have the last one resent to your mobile phone).