If you really want to think in English and activate your vocabulary, you need to read, write and talk in English as much as you can. In fact, you should've posted your query in English too. I don't think you can go by a time that anyone suggests. Each one has a different 'Sprachgefühl',and you may grasp words faster than others. If I tell you 6 months, you might not necessarily take that long. However, if you consider the following suggestions, it could help you a long way!
Try describing things in English...talk to yourself in English, irrespective of others finding it foolish. Describe the people going past you on the road. Try describing a festival or a recipe.
I have done this myself for foreign languages, and it has helped me a great deal.
This site is for idioms:http://home.t-online.de/home/toni.goeller/idi...
Also, have ready access to www.m-w.com and www.hyperdictionary.com
Having mentors/friends, who are native speakers could expedite your learning process.
Fluency and thinking in English will set in, as soon as you put the language to use.
Wish you loads of luck