Most manufacturers offer axles lengths that are 8mm longer than OLD, so 4mm overlap would be the defacto optimum. for forged dropouts. Many bicycles with stamped dropouts were equipped with QR hubs and a typical stamped dropout is ~3mm thick. so that would also be OK. Going below that, a lot depends on the condition of the axle end. For instance if there's a 1mm bevel, that 3mm overlap is only 2mm of actual bearing surface. I certainly wouldn't risk zero overlap and rely totally on the QR compression. One sudden jolt from hitting a pot hole or similar could cause the the wheel to unseat and jam against the pads or stays. Personally, I wouldn't go below 2mm of bearing surface.