Pregnancy test help

Hey how long can a urine sample stand before giving a false positive... i had a pee upstairs and dip 2 cheapies in and can see 2 very very faint evaps... brought the urine down stairs and dip in frer in and got a faint positive... anyone else had this... im worried the my frer is a false positive... top is one step from ebay suposed to be 10mlu but people say its higher middle is clear and simple 20mlu and bottom is first responce eary result... im 10dpo and afs due 15th xx