There is no definitive answer to this question as it depends on a number of factors, including the rower's age, weight, and experience level. However, general guidelines suggest that a good km rowing time for a lightweight rower (under 130lbs) is around 6:30-7:00 minutes, while a good time for a heavyweight rower (over 185lbs) is around 7:00-7:30 minutes. These are only rough estimates though and ultimately it comes down to the individual rower's abilities and goals.