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The duration of the marriage procedure highly depends on how long it takes to gather the required documents. You need a valid passport, a certificate of birth, a certificate of no impediment and maybe others too. Ask your local registry office (Standesamt) for a comprehensive list. Non-German documents need to be translated by a verified translator. Also you might need a legalisation which means that a German diplomatic representation (e.g. the embassy in your home country) will check your documents for authenticity and correctness. This procedure can take up to several months, so it's important to get in contact with Standesamt as soon as possible. They will tell you more about the exact procedure.
We received a lot of questions about marriage in Germany in general. That's why we wrote an article on our blog about it that summarizes the most important things to keep in mind. Please follow this link to check it out (scroll down for English version):