The eight-hour fast basically has two components:
It takes around two hours for food to pass through your stomach. If you fast for two hours, you should have an empty stomach. This probably decreases the risk of vomiting.
It then typically takes another six hours for food to pass through your small intestine, although this is much more variable. The further your last meal has moved along your small intestine, the more of its surface your drink of SN solution will have access to, and the faster the SN is likely to be absorbed (and the more likely it is that it will all be absorbed).
Personally, I think a full eight-hour fast is serious overkill. When I hopefully kill myself on Thursday, I will not fast for eight hours, because I know I'll get hungry to the point of discomfort (one of the drugs I'm on makes me eat almost compulsively). Instead, I will fast for at least four hours, hopefully around six. I think this is a level of discipline I can manage. I'll make it easier for myself by killing myself in a cheap hotel, with minimal access to food, which will require much less discipline than being at home. A two-hour fast is all but essential, and time in addition to that will increase my chances of success, but with diminishing returns, I think. Going from two hours to three is more valuable than going from three to four, which is more valuable than going from four to five, or five to six, and so on.