It Depends on Display
The horizontal distance between floating shelves is determined by the height of the items that are being displayed or stored on the shelf. When placing the shelves horizontally, allow room for the tallest item on the shelf plus space for visual effect.
Studs Limit Location
The vertical spacing of floating shelves is affected by the location of wall studs. Floating shelves' hidden brackets must be secured to studs. They cannot be secured with drywall anchors, or the weight of the shelf and contents will eventually pull the shelf away from the wall. Floating shelf brackets bear all of the weight.
Floating shelves can have as much or as little space between them as a decorator wants when it comes to horizontal placement. However, vertical placement is limited by the location of the studs in a room. Always secure floating shelves to wall studs under the drywall.