If you're taking a photo on shutter b/bulb setting/long exposure in the day time, will it turn out?

i get really mixed up with the technicalities of manual photography sometimes, but i want to know if i take a long exposure photograph i.e. keeping the shutter open for about twenty minutes in the daytime, will it turn out or would that overexpose the photograph? i don't understand how it couldn't, having that much light coming in and yet there are plenty of daytime long exposures on the internet.