For cardiovascular health, the American Heart Association recommends LISS cardio, 20 minutes minimum a day but realistically should be targeting 30 minutes (5-10 minutes to warm up). LISS involves a target heart rate of 60-65% of a person's max heart rate. The formula given by the American Heart Association is 220 - age = max heart rate. So for example, if you're 35 years old, 220 - 35 = 185 max heart rate. 65% of that is about 120 BPM.
Practically, I'd say a range of 120-140 BPM is a fine range to stay in for LISS. And of course, a heart rate monitor is the best way to assess if someone is hitting their heart rate range target while exercising.