Translation:The length is of approximately one hour.
As a native English speaker, I would not say, "the length is of approximately". Translation aside, if a person was asking how long a class exam might be, I would say either "It takes about an hour." or "It lasts about an hour." possibly in the case of a medical exam or test.
You can not translate literally. The "de" is needed in the Spanish translation, but we do not say "of" in English. Take this example: "The store is one mile from here" is "La tienda está a una milla de aquí". We don't say, "The store is at one mile from here", but the "a" is needed in the Spanish translation. Different word, yes, but same principle.
Here are ways of saying this, that sound normal to me as a native English speaker:
"The duration is about one hour."
"The duration is approximately one hour."
If I replace "duration" above with "length", then I feel freer about using "one" or "an" interchangeably with the word "hour". However, it is a bit strange to my ear to use "duration" here (which is a slightly more formal expression than "length") with "an hour", which sounds less precise than "one hour" and less compatible with the formality of the word "duration" (even with the accompanying qualifier "about" or "approximately").
All that said, more common expressions are:
"It lasts about an hour." (e.g. how long until something expires/finishes)
"It takes about an hour." (e.g. the length of time for a task to be completed)