From what I could gather, Anti-Emetics will increase your chance greatly although they´re not necessarily essential. That said, those that do it without usually also go through the entire fasting regiment. So 18-48h. Dani did (I think) and there is this other well documented case of another member who did so as well:
https://sanctioned-suicide.net/threads/survived-my-sn-last-june.52780/
Sure he survived but only because he was discovered in time. He still passed out and all.
Otherwise it helps reading through the
Fatal-SN attempts doc on the megathread and correlating all the cases, so as to hopefully draw your own conclusion. There are some pretty detailed ones on there, even one with an observer, who documented all of it.
So really, I think it really depends on how well your body is able to absorb things and keep them in. Body weight, mass and your metabolism would probably influence that a lot. Generally speaking it would make sense to me, that if your body weight was on the lower side, you would thus have a higher chance of getting through it without antis. But don´t take me up on that. I´m no professional.