Home Improvement Questions? Ask a Handyman for Answers
The short answer is twelve inches.
The 12 inches is from the bottom of the mantle down to the fire box . This should not be confused with the 6 inch spacing allowed on the sudes of the fire box.
National Standard Building Code Information
2114.3 Masonry-built Fireplaces 4114.3.10 - All combustible mantels and similar trim shall be kept at least 6 inches from the fireplace opening. Parts of the mantel assembly located along the sides of the fireplace opening which project more than 1 1/2 inches (38mm) from the face of the fireplace shall have additional clearance equal to the projection extension beyond 1 1/2 inches. Parts of the mantel assembly located above and projecting more than 1 1/2 inches (38mm) from the fireplace opening shall not be placed less than 12 inches (305mm) from the top of the fireplace opening.
This is a copy of the building code.
These are minimum standards for average size mantles.
Custom large fireplaces with oversize beam type mantles that extend out from the wall should be given extra consideration
and possibly placed a few inches higher.
and placed higher than the 12 inches at tge designers and owners discretion.