Visual inspection is actually a method used to collect data against an acceptance criterion.
Per PDA’s Technical Report 29 “Cleaning Validation” (2012) you only need a quantitative visual limit if visual examination is the only method used and visually clean is the only acceptance criterion. According to the report, if you are using visually clean as an acceptance criterion in conjunction with swab or rinse sampling, you do not need to quantify the visual limit and what “visually clean” means.
If you do need to quantify the visual limit, you can do so by spiking coupons with solutions at different surface densities (e.g. microgram/cm2 or ppm) and having a panel of qualified personnel determine at which level the residue ceases to be clearly visible. Distance, lighting, and viewing angle need to be considered.