I think it is important to find your limit. I tend to workout 30-45 min twice a day, 4-6 days a week. I know within time I will be able to increase that little by little, but for now that is my limit. Going above and beyond that has negative results.
Over doing it not only can cause physical injuries, but it can also lower your stamina and motivation. Both causing you to take time off from your fitness routines. The key here is consistency, not how much or how long!