In my scenario, all of current humanity on Earth and space is teleported to a whole new planet in a different universe with new laws of physics that mostly govern a magic system based on energy manipulation and different biology. Humanity is transported without anything from Earth, just with their own minds and clothes, and from then onwards, there are continuous wars and famines, resource wars and even more wars for over 100 years as new countries form and fall and fight over territory.
How long would it take for humanity to devolve into medieval level society and technology if the printing press is never reinvented and books about the old world are constantly burnt and destroyed from constant wars and the only method of knowledge of the old world being passed down is orally from one generation to another?
Edit: Due to stuff like no shelter, hard to find food and poisonous water, and of course, magic monsters that show up at night and are stronger than full grown African elephants, 7.5 billion people will die, only 500 million will remain after the first year. These 500 million would either be smart enough to trick the monsters, be able to group together and be able to make a base quick enough or would have enough mana to fight the monsters.