“All the episodes are 12 minutes long”?
No, each of the episodes is 12 minutes long.
If all the episodes were 12 minutes long, the entire series would take 12 minutes total.
(If you had 12 episodes, each would only be 1 minute long. What a teeny show!)
Whenever you write the word all, take a second look and see if you wouldn’t be better served by replacing it with each. If the stuff that comes after all the whateverdescribes just one of the group you’re discussing, you probably want the singular form each, not the plural form all.
It’s not a rule, just a suggestion. However, I’ll feel I’ve contributed positively to your life if I’ve managed to make you paranoid about using the word all.