I took pepto bismol (bismuth subsalicylate) for a stomachache a day ago, im now having black stool, how long will this last?

A day or so. It will last another day or so. It is from the stomach acid effect on the bismouth.
Not long unless the. Black is not due to the bismuth in pepto bismol (bismuth subsalicylate). Black stools can be caused by blood in the stomach which may also be your cause for your stomach ache. Please see you pcp and get checked before something more serious occurs.