As a foreign student you must prove adequate knowledge of the German language in order to study in Austria.
As a general rule, you will have to take a German language proficiency test - DaF ( Deutsch als Fremdsprache). Having passed the DaF you won't have problems entering any University. However, certain Universities and courses require a less formal proof of language skills, so check out the exact requirements directly with your University.
Let's say you need the DaF and have absolutely no knowledge of German, how long will it take you ? A tricky question - this depends on how much time you can spend on picking up the language. Within a couple of months (while actually living in Austria) you should be able to pass the DaF without problems. Remember: the best way to learn a language is having to use it.