That is, the amount of time you spend in the zone is the way you measure your workout, not in miles run, or the number of strokes per minute cycled or rowed. Varying your workouts, both in activity and zone, allows you to get multiple benefits from your training. Zones have specific numerical values, they are weighted. If you really wanted to fine-tune your training, you might choose to narrow the window to Zone 3. Use the following chart to determine the specific numerical values for your five zones.
Higher zones require less time in zone than lower zones.
But, as you move up to higher intensity zones, you need to decrease the amount of time that you spend in that zone, particularly in the top two, the Anaerobic and Redline zones. Zones are relative. Your five heart rate zones are specific to your maximum heart rate, not anybody else's. With two runners, each maintaining a heart rate of 160 bpm, one might well be in their 24 Threshold Zone and the other may be in their 22 Temperate Zone. It's all relative.