Assalamualaikum, does watching shows/movies containing things like magis, sorcery, and all the haram stuff, does it render your prayer invalid for 40 days when you watch it but dont believe it?

No because it is not real sorcery, but if it is soothsaying and so on becareful, do not watch such things. And shows and movies are full of that which is forbidden so this is not the only problem, you should quit all of this without hesitation.