It means that the person will return in an hour – although such a casual expression may indicate that the person may be away longer than that. Or less time than that ...

But it definitely does not mean that the person intends to leave in an hour's time.

It means that the person will return in an hour – although such a casual expression may indicate that the person could be away longer than that. Or less time ...

But it definitely does not mean that the person intends to leave in an hour's time.