When you treat your clothes at home with permethrin, it generally lasts through six washings or so. You can also send you clothes in to the Insect Shield folks and have them perform a permethrin treatment that will last for about 70 washings (they claim). I do it before each season for the clothes I'll wear that spring/summer/fall.

http://www.insectshield.com/IS_Your_Own_Clothes/default.aspx