The average person can empty their stomach in 4-8 hours (depending on the last meal) which is why the PPH advises fasting for "several hours" before SN. If you have a slow emptying stomach, I would push your fast to 12 or even 24 hours if you can. I am no GI expert though, so I do not know how your system will handle SN. The one shining grace is that it is dissolved in water instead of a plain solid so, in theory, it will be easy for your system to move along.