The swimmers that are going to make it to their destination will get through the cervix in the first few minutes. Any semen left in the vagina after about 20mins max only contains dead/dying/duff sperm. After 20 minutes you are safe to get up! By leaving it so long to go for a wee, you are risking a cystitis infection, which would seriously interfere with your swi! It is good hygiene to have a wee after having sex and it will not prevent you conceiving.