A skin fade looks its sharpest for about ten days. After two weeks the skin at the nape is covered and the blend between the guards starts to soften.
Most regulars come back every two to three weeks. If you only want the sides and the neckline cleaned up, ask for a fade tidy: about 20 minutes and less than a full cut (example prices).
Tell the barber the guards you had last time, or show them this site’s price list: the chips on each cut are the guards we use.