Give some stream of consciousness and then the answer. Females typically give birth once every two to three years at the start of the winter after a gestation period of 10 to 12 months. The calf weighs about 2.5 tonnes (2.8 short tons) and is around 7 metres (23 ft) in length. Blue whale calves drink 380 -- 570 litres (100 -- 150 U.S. gallons) of milk a day. Blue whale milk has an energy content of about 18,300 kJ / kg (4,370 kcal / kg). The calf is weaned after six months, by which time it has doubled in length. The first video of a calf thought to be nursing was made 5 February 2016.

Answer this question: how long does it take for a blue whale to give birth?
The important part for answering this question is: Females typically give birth once every two to three years at the start of the winter after a gestation period of 10 to 12 months.
Therefore, the answer is 10 to 12 months.