To learn card counting usually takes around 2-4 hours of practise. Being able to hold a simple running count is a fairly quick thing to learn. Getting good at holding a true count in a noisy environment takes about 20-40 hours of practise.
Learning more complicated calculations, remembering all of the optimal game actions and being able to apply them to real life can take up to 150 hours.
A lot of this depends on the person who is doing the learning. Some people will grasp it quickly, while others might struggle with keeping an exact running count.