A German car A/C would need to be recharged when the system no longer has enough refrigerant to cool the cabin efficiently. In a healthy A/C system, refrigerant is not consumed like fuel or engine oil. It circulates through a sealed system over and over. When the level gets low, it usually means the refrigerant has escaped due to a leak, a failed O-ring, a worn compressor seal, a corroded condenser, or a damaged hose.
Commonly, refrigerant loss occurs slowly over several years, but the cause should still be checked. A recharge restores the correct refrigerant level, but it should also include leak testing. Simply adding refrigerant without finding the cause of the leak may only provide temporary cooling and can allow bigger A/C problems to develop.