Great answer Joan and great guidelines that everyone should follow!
One of the biggest mistakes I see people make is trying to do too much in the beginning. Consistency is much better than how much you do. Who cares if you do 2 hrs a day if in a month or two you no longer are exercising.
It is much more productive to do 20 to 30 minutes in the beginning and be consistent with it and gradually build it up over time.
The amount that you should do is very individualized but my point is consistency is more important than the time. Too much exercise in the beginning can be too much for most people and the rate of burnout can be quite high. However, a moderate amount of exercise that someone enjoys and doesn't cramp their style causes a greater likelihood of staying with the program.
If someone exercises 30 minutes a day for 6 days a week they will exercise a total of 3 hours per week or a bit over 150 hrs per year. Doesn't sound like much huh?
If someone is off and on and has an all or nothing attitude and exercises 2 hrs for a few weeks and lets say like hits and they stop because 2 hrs just does not fit into their schedule or desire. Lets say someone trains hard for 2 months and then takes 4 months off. We know a lot of people like this.
In a year they still workout 150 hrs but which one do you think is more productive? It is the person who is consistent because this person is continually doing something (although limited) and when you are exercising your likelihood of eating well also increases.
There is nothing wrong with 1 or more hrs of exercise per day, my point is consistency is the true key in any fitness program.