"He plays for an hour."
Translation:वह एक घंटा खेलता है ।
The thing is that unlike जाना and आना, खेलना is a transitive verb and can take a direct object. However, the direct object should usually answer the question "What is being played?" Eg: वह क्रिकेट खेलता है.
How 'एक घंटा' can be a direct object to खेलना making 'वह एक घंटा खेलता है' grammatically correct is not clear to me. I would prefer 'वह एक घंटे के लिए खेलता है' [In sentences like मुझे एक घंटा दो, you can see how 'एक घंटा' clearly answers the 'what' question and therefore fits]
Sidenote: घंटा is also a slang term in colloquial Hindi which means 'nothing' with sarcastic undertones. So, 'वह घंटा खेलता है' would then mean 'He doesn't play anything'.