How many times can one exercise in a day?
TLDR. Depends on the intensity and over training affects you in later life.
It very much depends on your fitness level and your recovery time, I started from scratch this year doing 4x3 minutes on a rowing machine at home. A couple of months later I was doing 75Km, 4 hour cardio workouts, 45Km bike, 15Km rowing machine, 10Km elliptical and 5Km total walk to and from gym.
A couple of hours after getting home and eating, I started to feel antsy and ready to do the same workout again I did not do it because I would need to consume a massive amount of calories just to stay the same weight. I would instead go and walk the dog for a few Km, you can train too much.
It is important not to over train, something I wish that I had learned in my teenage years. I am 45 next month, awaiting knee surgery that results from excess training from about the age of 12, my other knee is shot as is my left ankle. All this from excess training, I used to regularly pass out whilst I was training.