Howard Brickman of Norwell, Mass.-based Brickman Consulting answers:
For solid tongue-and-groove wood flooring, the wear-layer thickness is the depth above the tongue and groove. With engineered wood flooring, it is a combination of wear layer thickness in conjunction with the position of the tongue and groove. Note that at ⅛ inch above the groove edge over the tongue, the groove begins to split or splinter when sanded. A square-edge floor, with no tongue and groove, can usually be sanded to within ⅛ inch of the subfloor if it is well-fastened with no up or down movement.
How much material is removed during a typical sanding? Factors that can affect how much wood is removed include: flatness of the surface (which usually means the flatness of the subfloor); stains or other discolorations that must be removed; and changing the stain color from dark to light, to name a few. The skill of the person running the sanding equipment also affects this—an unskilled person may over-sand, with too much material removed. The average amount of wear surface removed by a skilled sand-and-finish pro over a flat subfloor during a total resand would be 1/32 to 3/64 inch. So a typical solid ¾-inch strip floor would typically have up to five sandings in an ideal world.
You can tell that a floor cannot be resanded by observing "shiners" between boards (see the photo below)—blind nails that have been exposed by previous sandings. Or you may spot splinters along the groove edges where the top of the groove is so thin that it is broken or even missing, exposing the top of the tongue of the next board. It can be helpful to determine the thickness of the remaining wear layer by placing a thin putty knife between the boards (if there isn't finish obstructing access). This should be done in multiple locations. To determine how many sandings it has left, subtract ⅛ inch from the actual measured wear surface and divide by 3/64 inch.