I wish there was an easy way to answer this question, but every body is different.
What I can tell you is that cycling is EXTREMELY easy on your body in comparison to weight training, Zumba, HIIT Training, etc. It's incredibly low impact on your back & joints. When you think about other types of training; running, jumping, sprinting, hitting, lifting, they all involve some type of impact, which over time, can lead to some painful issues.
Cycling however, can be done until you're 100 years old, lol! That's not to say it doesn't come with it's own issues, I'll get to that shortly.
Now, when it comes to how YOU are feeling, only you can decide if it is too much for you. If you find yourself extremely fatigued, experiencing headaches, muscle aches, you might be over doing it. It's OKAY to take a rest day! However, if it's day 5 of cycling, and you're still feeling energized, motivated, and excited to take on a 45 minute ride, then by all means DO IT!
It's like anything really... too much of something is not always a good thing and can lead to complications. Horror stories about hospitalizations from getting rhabdomyolysis after a cycling class are most definitely real, but are extremely rare and often times occur due lack of proper hydration and/or nutrition and/or pre-existing conditions.
Long story short, if you feel great, get on your bike and ride! If you feel like you could use a break, take a break! Me & your bike will be waiting for you when you're ready!