Lynxx's scent will stay for diffrent lengths in different areas. Areas Lynxx frequented most and are least used by Luna and Midnight will remain the longest. As Luna and Midnight start marking everything, Lynxx's smell will be pushed out of the way by the their smells, until it is gone. It can take weeks in the areas Lynxx was in the most.
As for remembering Lynxx, who can say? Cats can't tell you if they remember their old friends or not, but I am tempted to believe they have the ability to mourn a friend for a really long time. Since Luna and Midnight are both so young, and since they are much closer to each other than either of them was to Lynxx, I'm not sure if they will remember Lynxx for much longer than his scent stays around.