about 36 hours, and I would not recommend it. I normally function really well on 4-5 hours of sleep, and even if i do plan on studying ALL night, i find it better to study til like 4-5 am and sleep for 2 hours, wake up at 7, and review the material a few hours before the test. Even those 2 hours of sleep, albeit little, are better than NO sleep at all. I would highly recommend this as opposed to staying up consecutive days.
I did have a friend who was the manager of whataburger, and dual majored in Chem + Bio and he would constantly go without sleep. He regularly was up for 2-3 days straight and I have no idea how he functioned. Oh wait yes I do, he called it a study cocktail, containing redbull, monster, and a little bit of adderal with a shot of 5 hr. 😱 Would not recommend this because I cant imagine it being remotely healthy.