The number you will be quoted is twelve weeks. That figure is real, and it describes one level. From no German at all, B1 is three levels — A1, A2, then B1 — and the honest planning figure is around 30 weeks of consistent class time, or roughly 150 to 180 contact hours.
Here are the assumptions behind that, so you can argue with them rather than take them on trust. A motivated adult, four class hours a week, three to four hours of independent work between sessions, a group of eight to fourteen, and no gap between levels. Break any one of those and the timetable stretches.
The most common thing that breaks it is the gap between levels. A cohort finishes A2 in April, the next batch starts in July, and by then a meaningful part of A2 has gone. Continuous laddering is worth more to the timetable than intensity is.
If your deadline genuinely cannot accommodate 30 weeks, the conversation to have is about scope rather than speed. A2 by the date, with B1 following, is a plan. B1 by a date that does not fit is a sitting fee spent on a fail.
We would rather tell you that at the brief stage than sell you the shorter answer.