By law you are granted 25 days vacation per year, assuming you have worked enough to "earn them", generally you get 12.5-13.x% extra salary per day worked that goes towards your vacation.
The law also says that you have a right to 4 weeks uninterrupted vacation during the June, July and August. At least two months before the (requested) vacation you must get a definitive decision. The employers are not allowed, by law, to change their mind about granted vacation time.
I believe I saw a study that recommended shorter, more frequent, vacations as the best way to recover from mental fatigue but I assume that there is a lot of opposing opinions on that matter.