How do know when it's safe to progress with your cardiovascular system training?

It's safe to progress with your cardiovascular training when you can complete your current workout intensity and duration without excessive fatigue or discomfort. Additionally, if you're consistently recovering well and not experiencing signs of overtraining (like persistent soreness or decreased performance), it's a good indicator to increase intensity or duration. Monitoring your heart rate during workouts and ensuring it stays within target zones can also guide you in determining readiness for progression. Always listen to your body and consult with a fitness professional if unsure.