There were many weapons used in medieval times, and it was important for a knight to know how to use those weapons.
For some knights, training was a few months, or 14 years. All knights were required to have a high level of skill, and expertise in combat.
There were also different ways of training for different types of weapons. For a bow you would have shooting lessons, but a dagger you would have lessons on how to jab, or where to stab.
It was very important to have multiple knights that had different areas of expertise in combat, because there was a lot of fighting for different lands, or cities, or even castles. So a good kingdom relied on good knights.