I was asked the perennial question about learning two languages, and also about how long it takes to learn a language, at our Forum at LingQ. Here is what I had to say.
Your enthusiasm is great, and a guarantee that you will succeed. Each person has to find his or her own way in language learning. Different people enjoy different learning activities. The most important things is to keep at it, and to put in enough time to succeed.
I find it easier to focus on one new language at a time. Once a language is at an intermediate level I can mix it with other languages.
If you are most keen to learn German, I would stay with it for now, for at least 6 months. You can always dabble a little in French, here at LingQ for example, since it is much closer to Spanish. But I would keep my main focus on German. Maybe 80% German and 20% French. You will find that you will be naturally more attracted to one or the other.
I would not worry too much about German grammar right now. I would get myself a small book explaining some of the main grammar issues that you will come across in German. You can also search for German grammar explanations, summaries, as well as verb and noun/adjective tables on the web. Use these to refer to from time to time when you are interested. Don’t expect to remember much. Don’t expect to use the grammar correctly. Focus on listening and reading. Don’t worry if a lot of the text is unclear, even after you have looked up all the words. It takes time for your brain to get used to the language. When you have enough exposure and a larger vocabulary, the grammar rules will become easier to remember.
As to how long it will take, I cannot say. Try to focus on enjoying your learning rather than on how long it will take. Assume that it will take a long time. I think it might take a year before you can comfortably converse in German, but it could go faster if you have a lot of time to spend on it.