As a Personal trainer, I am faced with this question all the time. Firstly it depends on your goals, if you want to lose weight and tone up then the simple answer is: as many times as possible. But before you jump the gun and assume this means heavy training every day. That is not physically possible and will be counter productive. If you want to build muscle mass, the answer is basically the same, however the training is different and recovery time is crucial to progress.
It is recommended that sedentary individuals, or those that are usually inactive, should try and achieve at least 30 minutes of moderate activity each day, this can be broken up into 2 rounds of 15 minutes or 3 rounds of 10 minutes each day. These activities can be such things as: Walking at a brisk pace, gardening, washing the car, housework or swimming.
In addition to this some adjustments will need to be made regarding diet and caloric intake, you can’t compete with what you eat! Eliminating processed foods, sugary foods, and fatty foods is a start, but nutrition goes way beyond the scope of this post, check out this post : Weight loss: The right way especially points 5 & 6 to get a better understanding of nutrition for weight loss.
Progression is another factor often taken for granted with beginners. With time, your body will become used to the level of exertion you place upon it, and a plateau in weight loss or progress will occur. In order to progress to the next level you have to increase the workload, by increasing the intensity or duration or both! But don’t be too hasty, small increases over time will yield great advances, for example: I have a client senior in years but spritely in nature, who was initially apprehensive at the thought of going to a Gym, as you would expect. He is now thoroughly enjoying the experience and was once worried about letting go of the treadmill grab rails while walking. He is now running for the first time, unaided in 40 or so years. It has taken time to get to this level but it has been more than worth the wait, and thoroughly compelling and enjoyable for me.
So, in summary: If you are new to exercise, try and do a little each day and progress gradually. If you have reached a plateau or wish to build muscle, allow time for recovery (muscle soreness/stiffness to subside) before training those particular body parts again. Give it a try, what have you got to lose? Apart from a few pounds of weight.