Hi there, there is no exact height, however in general for a linear fireplace I would say to have the top of the firebox be no more than 40" off the floor.

I agree with Chimera, and that is a good general rule. However, it depends a bit on the use of the room, and whether the fireplace will be seen more from a seated position or standing position, from in bed, etc.

In a foyer, at the end of a long hallway, in a bedroom or near a dining room table (where you would have to look over the table to see it), and where the fireplace is the ONLY thing on the wall, I recommend the top at about 60" off the floor, so most folks would still be looking down into it, but it is a better focal point than it would be close to the floor.

If it will have a TV or artwork over it, and it is the focal point of a seating area, I recommend placing the bottom edge of the viewing area about 18" off the floor, although there aren't any hard and fast rules.

The one important thing to avoid when considering the minimum height is having it so low that it looks like a fish tank on the floor.

Here are a couple of nicely balanced examples in seating areas: