In 24-hour clock, for certain point of time, the correct way is 06:30. The first pair of digits corresponds to hours, and the second pair (after colon) to minutes. Thus, "06:30 hours" is incorrect, since the second pair corresponds to minutes, not to hours, and "0630 hours" is even more confusing. Just imagine another example, 1104 hours, that is 46 days (1104/24).

We can write word "hours" for quantity, total, or sum of hours, like in answering question: "How many hours?"