If you're using TrainingPeaks software to train for your Ironman event, you likely are using their Performance Management Chart, (PMC). Chronic Training Load, or CTL, is one of the key metrics in the chart. It's the long term Training Stress Score, (TSS), average. How long is long term? The default setting is 6 weeks, as a 42 day rolling average.
So when you upload your data from your ride, the software reads the samples from the data and compares those with your FTP to calculate how stressful the ride was, and then takes that score and averages it with the previous 41 days. I actually use the software to keep a separate PMC just for cycling.
The question many have when they train for an Ironman event is, "How high should my bike CTL get?" The answer to this question is, (like almost all training questions), related to your goals for the event. For example, if your goal is to simply finish the race, then a lower CTL is fine, but if you're trying to qualify for Kona, or win your age group, then you likely need a much higher CTL. If you're a pro, trying to earn a paycheck, or win the race, you should probably be even higher than what the top age grouper is achieving. If you're trying to win Kona as a pro, then you likely need an even higher CTL.
If you're looking for a bike-only CTL value to achieve in your training, the number is likely going to be related to your FTP value. Why? Because usually, the higher the FTP of the athlete, the higher the performance goal. Here's a chart for you to use based on your goals, and what I have found to be the tendency among the different types of athletes.
With sharing this chart, there are 4 important things to keep in mind...
1. This is just a guideline for PEAK CTL VALUES, mostly for seasonal planning purposes. Because of that, there will be people who don't fall into these guidelines, but I do find a majority of athletes do. I don't find there is much need to list a mid-pack athlete, since they vary the most in terms of background and training styles.
2. Your swim, bike and run skill will also play a role in whether or not you would achieve these goals. Again, this is a guideline for your bike training, knowing how much is enough, or a range of what might be enough.
3. The course an athlete races for their goal event is a big determinant of the value one should achieve as well. For the Kona Qualifier Pro and Pro Podium at Kona, that is course specific to Kona. The Age Group Kona Qualifer is general for all courses, since age groupers qualify at many different individual races. There is a big difference between qualifying at Ironman Lanzarote and Ironman Florida.
4. Your FTP will likely, (and should), improve throughout the season. For some it will improve more than others. This means your initial CTL value goal will likely change a little, so give yourself a range, or be prepared to adjust it as the season goes.
How can you use this information? Look at the end of your season, what your FTP was, the peak CTL value you achieved, and what your goal was for the event, and see how well you lined up with this chart. (Please share in the comments your results as well). You can then use this information to better assess your training, set new or different bike CTL goals for your upcoming events, and use those goals to help motivate you in your training.
Lastly, CTL doesn't win races, performance does. No awards or Kona slots for who had the highest CTL. Don't get hung up on CTL. Make sure you are seeing the performance gains you want in your training first and foremost. It's all about balancing training stress, this is just a guide to use and better understand your training, so you can improve it.