訳:The minimum time is an hour.
That does not sound correct in English. This sentence is more properly written as "The minimum period of time is one hour." The subject is "the minimum period of time" and it is described as being "one hour" by using the copula "is".
One may say "the minimum is one hour", however. In this case, "minimum" would refer to the "minimum period of time", as above.
Thank you for your comment to my post. However I don't understand what you wrote. I suppose this Japanese question asking about the chioce among mamy hours options. For example ,In a game station, there are "one hour" set of playing time, two hours play sets, and so on. And the shop worker explains the player ," 「The minimum hour is one hour.」.