I had a bilateral mastectomy in April. Mapping of sentinel lymph nodes began first thing in the morning. The technician was wonderful. The first scan was 15 minutes after dye injection. She explained that it can be much slower, that they had a record of 3 hours. I beat that record and still just a possible "hint" on scans but nothing definitive. I was stressed and concerned as my surgery time arrived. Ultimately the hint was enough and a geiger counter used in surgery confirmed the location of the sentinel nodes.