How long should I limit my workout to?

No perfect answer. No perfect answer... Remember damage occurs during the work-out, growth occurs with healing. Without adequate rest you won't get growth. I generally tell people not to work the same muscle groups more than 2-3 times per week, and if intensely training, possibly less. Then again, cardio and even light circuit training can be done daily.
Level of fitness. Great question you should clear activity with you doc, then gradual increase in exercise intensity can be done by increase time or speed (running, cycle, swim) you need about 30 min per day at a training heart rate at least 5 days per week. Recovery should be 100% by the time of next workout. Gradual is the key to avoid injury and over training.