There are minimum code requirements. I think code is " 16" to combustible mantle from top of firebox."

Clearances from a masonry fireplace to a mantel, as required by code, define only your minimum, which is a little complicated because it is a sliding scale depending not only on the distance above the opening but the depth of the shelf.
Manufactured fireplaces are defined solely by the manufacturer's installation manual.
Having said all that, most folks would not want their mantel at the minimum height.
Average mantel height is 52-58" above the floor.

The mantle in this photo measures 54" above the fireplace.

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