The Higher the Better
The first level to be taken seriously is the B2 level. Your aim should be C1 and from there you should just pick up still unknown words and structures on your own and be able to look them up yourself. C2 is mostly needed for higher educational work and is almost native level.
Why B1 Then
The first exam I offer is the B1 as the other two can be reached after a week or two and don‘t prove much. With B1 you can actually have a long conversation about a admittedly yet limited scale of topics but people won‘t switch to English that often anymore which will boost the further learning process significantly. Passing this landmark is very important for you motivation and level of commitment. The feeling after you have passed this first crucial level is invaluable. It comes after a perfect amount of days, where you are still fresh and yet already confident. B2 would be too far away yet and B1 will give you more than enough boost to continue until the first real challenge: The B2 exam.
Become a Citizen
The German government thinks B1 to be enough to get along well enough with the Germans in case you are looking for citizenship status. They say understanding 2,700 words is enough. You can find a complete list of them here. But please don’t learn any list as it would be a waste of time. But going through it in a calm half hour just to see how many of the words you already understand might be interesting.
The Challenge is B2
B2 is the first level that means that you can speak really fluent and understand 80% of the situations that you find yourself in repeatedly.
You can prepare yourself for the exam and even see a sample of the language level here. Scroll down to „Prüfungsteil Sprechen (Mündliche Prüfung) direkt ansehen“ for the video.
How long Until…
In an intensive class you would need about 720 lessons to get to B2. That are nine months of 3 hours daily (not including homework). Costs vary from 1,800 to 9,000 for classes. Private lessons might require a bit more funding. Just for the very eager among you: C1 means another 240 lessons or three months or 600-3,000€.