The Australian government currently recommends that everyone undergoes 30 minutes of exercise per day which is 3.5 hours a week. It is important that everyone undertakes this, and even when you have an acute type of injury you should be trying to find a way to do general exercise as much as you are able, the options are walking, swimming, cycling, tennis etc.
As for the exercises you have been given for your low back pain or other problem. You should do them as prescribed by your Physiotherapist. The most important thing about exercises is that you should have had them explained to you, the reason for doing them and then if they are the right exercise, they should make a difference to your pain or problem. When you have exercises that do all of that, you will want to do them.
BUT I know that not many people do continue to do them though, well not regularly anyway. The next thing I say is, if the exercises are working, remember how to do them and why you are doing them. Then if your problem comes back, you can easily do those exercises again because you have remembered them and your have remembered why you should do them..
How often do you need to do them to keep your problem away long term? In this case I would say a couple of times a week, include them in your gym or general exercise program, after a walk, a swim, or at the gym. If you are having trouble with that join a class, at the clinic, yoga; a class that works on the finer points of posture and motor control so that you can work on your problem and create a long-term solution of prevention.