For class types, the theoretical complexity is O(n). The destructor is called for each element. Of course, it's up to the implementation to adhere to the observable behavior, so if the destructor is a no-op or the behavior is the same as with just marking the whole chunk as freed, the complexity could be just O(1).
For primitive types, the compiler will likely just release the whole chunk of memory at once, thus the complexity O(1).
