I have a question that relates to untapping a creature enchanted by Charmed Sleep. It says "Enchanted creature doesn't untap during its controller's untap step".
Does this apply to the following controller's untap step, or for any untap step?
(That is, does the enchantment last for one round or forever?)
It lasts permanently (as long as it's enchanting the creature). It simply states something which is true, so it will apply to all untap steps (of the creature's controller), not just the following one.
"Enchanted creature doesn't untap during its controller's untap step." is a static ability.
113.3d Static abilities are written as statements. They’re simply true. Static abilities create continuous effects which are active while the permanent with the ability is on the battlefield and has the ability, or while the object with the ability is in the appropriate zone. See rule 604, “Handling Static Abilities.”
So the rule-modifying continuous effect it creates will last until the Charmed Sleep leaves the battlefield.
Have a look at Crippling Chill. It states (emphasis mine):
Tap target creature. It doesn’t untap during its controller’s next untap step.
Here you can see how long this effect lasts. Without the "next", it simply means that whenever that player has their untap step, the enchanted creature does not untap.
