Certainly, you can break down cycling training zones based on rate of perceived exertion (RPE) using a 10-zone system. RPE is subjective and depends on how hard you feel you are working during a ride. Here's how you can define and sustain each zone in a 10-zone RPE system:

1. Zone 1 - Rest: This is a complete state of rest, such as when you're not cycling at all.
2. Zone 2 - Very Light: You can sustain for all day, or until you fall asleep. It feels effortless, like a leisurely ride or warm-up.
3. Zone 3 - Light: You can sustain  for a long time, typically several hours. You're still comfortable and conversational at this effort level.
4. Zone 4 - Moderate: Can typically be sustained for 1-3 hours. You feel like you're working, but you can maintain a conversation with some effort.
5. Zone 5 - Somewhat Hard: You can typically sustain efforts for 30 minutes to 1 hour. You're noticeably working, and conversation is challenging but possible in short sentences.
6. Zone 6 - Hard: You can sustain  efforts for 10-30 minutes. You're working hard, conversation is challenging, and you can only speak in short, fragmented sentences.
7. Zone 7 - Very Hard: Efforts are high-intensity and can usually be sustained for 5-10 minutes. Conversation is very difficult, and you're breathing heavily.
8. Zone 8 - Extremely Hard: In this zone, you can typically sustain efforts for 2-5 minutes. Conversation is nearly impossible, and you're approaching your limit.
9. Zone 9 - Near Maximal: Efforts are near maximal and can only be sustained for 30 seconds to 2 minutes. You're working at or very close to your maximum capacity.
10. Zone 10 - Maximal:  All-out, maximal effort that can only be sustained for a few seconds to 30 seconds. You're pushing your limits to the absolute maximum.

These durations are approximate and can vary from person to person based on fitness level and familiarity with high-intensity efforts. It's important to listen to your body and adjust your effort level based on your RPE. Training in specific RPE zones should align with your training goals and periodization plan. Consulting with a coach or experienced cyclist can help you tailor your training to your individual needs and objectives.

​In a 5-zone heart rate system commonly used for cycling training, the duration you can sustain each zone will still depend on your fitness level and goals, but here are general guidelines:

1. Zone 1 - Active Recovery: You can sustain Zone 1 for extended periods, potentially for hours. This is typically below 60% of your maximum heart rate (MHR) and is used for easy, recovery rides.
2. Zone 2 - Endurance: Zone 2 can be sustained for a long time, typically several hours. It's usually around 60-70% of your MHR and is excellent for building aerobic base fitness.
3. Zone 3 - Tempo: Zone 3 can typically be sustained for 30 minutes to a few hours, depending on your fitness level. It's around 70-80% of your MHR and is useful for improving aerobic capacity and muscular endurance.
4. Zone 4 - Threshold: Zone 4 efforts can typically be sustained for 10-30 minutes. It's around 80-90% of your MHR and is excellent for improving your lactate threshold and tolerance for higher intensities.
5. Zone 5 - Anaerobic: Zone 5 is high-intensity and can usually be sustained for 1-5 minutes. It's around 90-100% of your MHR and is used for short, intense efforts to improve anaerobic capacity.

These durations are approximate and can vary from person to person. Well-trained athletes may be able to sustain efforts at the upper end of these time ranges, while beginners may have shorter durations in each zone. It's crucial to monitor your heart rate during your rides to ensure you are training in the intended zone and to make adjustments as needed based on your fitness progress.
Remember that training in specific heart rate zones should align with your training goals and periodization plan. Always consult with a coach or experienced cyclist to tailor your training to your individual needs and objectives.

Most athletes do not go slow or easy enough on "easy" days, thus not allowing them to give it their all on the "high intensity" training days. 

Resting Heart Rate (RHR):

Resting Heart Rate (RHR): This is your heart rate when you are at complete rest. It's usually between 60-100 beats per minute (bpm) for most adults.

ZONE  1

aka: recovery/easy
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Used to get your body moving with minimal stress and exertion. This zone might be used for an easy training day, warming up or cooling down.Warm-up Zone (50-60% of MHR): This zone is used for light warm-up exercises. It's a low-intensity zone that helps prepare your body for more intense workouts.

ZONE 2

aka: Base, Fat Burning Zone

Used for longer training sessions, you can sustain this basic-effort zone for many miles, yet still chitchat a little bit with your workout partner. This zone is often recommended for fat loss. It's a moderate-intensity zone where your body primarily burns fat for energy.​

ZONE

ZONE  3

aka:tempo, aerobic, endurance zone
​
This is a zone where you push the pace to build up speed and strength; conversation is reduced to single words. This zone is ideal for improving cardiovascular endurance. It's a moderate-to-high-intensity zone where you can sustain activity for an extended period.

ZONE  4

aka: lactate threshold, Anaerobic or Threshold Zone
​
In this zone your body is processing its maximum amount of lactic acid as a fuel source; above this level, lactic acid builds up too quickly to be processed and fatigues your  muscles; training in this zone helps your body develop efficiency when you’re operating at your maximum sustainable pace.  In this zone, you are working at a high intensity, just below your anaerobic threshold. It's useful for increasing speed and improving lactate threshold.

ZONE  5

aka: Aaerobic, Redline or Max Effort Zone 
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This maximum speed zone (think closing kick in a race) trains the neuromuscular system—your body learns how to recruit additional muscle fibers and how to fire muscles more effectively.  You can only sustain activity in thi zsone for a very short time before fatigue sets in.