(rofl) Well - it depends on many circumstances - the decomposition of the body begins, basically, immediately after death (intestinal and other bacteria live and function), but its speed depends on many factors ( temperature, humidity, environment ...), so your question does not have a clear answer. In addition, even the penetration of "stink" outside the apartment does not have clearly defined factors (such as the tightness of the front door). : o But if you haven't seen your neighbor "for a while" - it wouldn't be obvious to call for help - what if he may stink but still lives and needs help ???