I've heard that the Pokémon attacks for every time that you tap the screen. So if you tap three time with bite then you'll attack three times and in total it will take up 1.5 seconds. FYI the defending Pokémon attacks every 2.5 seconds so with an attack like bite you can either attack four or five times ( the reason that I say three or four is because I'm not sure how dodging is taken into effect) due to bites 1/2 a second cool down. I hope this was helpful sorry for making the answer so long.
EDIT: tap however many times you wanna attack.