There is an ideal time of 1 year to learn German language from A1 to C1 level. But one has to study hard. you should practice everyday. Daily revision is equally important. Only Classroom learning is not sufficient to learn German. It applies to both - Online or Offline German Class.
There are students who are fast learners and slow learners. These are two broad catagories of learners. What type of learner you are?
Identify your strength and weakness. It is very important to do self analysis.
in 10-12 months one can be fluent in German, if one practices daily 3-4 hours apart from classroom teaching. This is not difficult. But if you can not do this much of study everyday, than it is difficult to achieve this goal.