It depends on how long it takes the method to work, as well as body decomposition based on the environment (such as being really hot; having AC blasting helps, even if it's not hot). The body may also release urine or excrement, so it's good to plan for that as well, such as laying down a plastic sheet.
I don't know how long N takes to work, it's usually within minutes, but I don't know the max amount of time.
SN could take up to four hours, so I would probably plan for a couple of hours extra, but that's just a personal preference.
Hanging could take up to half an hour, so I would again plan for a couple of hours extra, because I wouldn't want there to be any chance I could be revived.
Edit: I agree with @restingspot about leaving extra time in case one survives and needs to clean up. Members who have attempted hanging have reported urinating and defecating. With SN, one may vomit.