My plans are all bungled up now and I'll have to take SN while others are home, and I'll do it at night. My plan is midnight, so everyone's asleep. I don't expect my mom would become concerned till late morning (11?).
I guess it depends on (a) what seems like a natural hour for you to retire, (b) how long it would be before someone might get worried and come knocking. I read from an older man on the Derek Humphry's/Final Exit blog that he drank two bottles of N at 8pm, and his wife found him at 3am, and he survived (barely, but full recovery it seems). He's an outlier at 7 hours, but it seems wise to leave as much time before possible discovery as possible, just in case.