The CDC recommends 2 hours and 30 minutes of moderate physical activity a week which is brisk walking. So if that's what you want to do on a treadmill that's fine. Or you can do 1 hour and 15 minutes of vigorous physical activity per week. That's jogging and faster. That sounds like quite a lot and it is. You can do them in blocks of 10 minutes the CDC advises, if that's all you can do.
So if you can do 10 minute chunks then the vigorous exercise is possible. If you aim to do brisk walking then that is 2 hours and 30 minutes per week is quite a tall order. That equates to 30 minutes a day, 5 days a week. Vigorous activity is 15 minutes a day 5 days a week. Vigorous activity is easier to fit in your schedule as you could do 45 minutes in a day and that's half your allotted time.
What is your fitness aim?
How often you should run on a treadmill is also dependant on what you want to get out of it. If you're walking or running for weight loss then you can do 3 days a week. This will enable you to lose weight. This is the same for vigorous activity. The reason is it's not recommended to do all of your physical activity in one go, it doesn't work that way, you will not benefit or progress with it.
If you're exercising for fitness then the days spent exercising are pretty much the same (at least 3 days a week). The CDC says your health benefits by moderate physical activity for 2 hours 30 minutes per week but if you can do 5 hours a week you will get even greater health benefits. That is definitely something people will find difficult.
If you're training for a race then you would need to do one of your runs per week outside. This is to prepare you for the difference in running outside to running on a treadmill. Otherwise at least 3 days a week remains the recommended aim. It's this amount of time that will enable your exercising to progress as you extend your fitness levels.
Progression is the key
The exercises the CDC recommends they know can't be achievable immediately. You're going to have to work up to that amount of exercise time. For vigorous exercise that progress to 30 minutes per day I recommend using our couch to 5k on a treadmill. It's been designed for people who have never run before or for a long time and will get you started.
The 2 hours 30 minutes of brisk walking could be done over 3 days if you work up to an hour on 2 days and 30 minutes on the 3rd. Running can be done the same way for 1 hour 15 minutes. You see you have to work and progress to reach it. While you're doing that you can take it further. You will have been exercising for around 8 weeks to reach this point so you may as well see how far you can go.
If you're doing the same thing once you get to the recommend physical activity time you will soon get bored. So look to exercising for a longer period of time and going further. The reason we say 3 days a week is because that's the optimum regularity you need to exercise for your body to adjust. Your body will be able to progress to a new level if you exercise at that frequency.
To summarize I recommend you should exercise on a treadmill 3 times a week or more. This way the CDC targets for physical activity per week are reachable. Remember not to just get on the treadmill and just walk and look at the time and leave after so many minutes. It's the quality you need to think about. This is how you progress to running further and faster.
There are so many benefits to getting this exercise that it makes it worthwhile. It's not recommended by a government organization for nothing. It's a life extender and not just a new year's resolution. If you can stick with it you reduce the chance of getting so many life limiting diseases. These are diseases that make your life miserable and can get you at an early age.
You reduce your risk of getting diabetes and many cardiovascular problems that can arise. For the treadmill make sure you get some decent walking or running shoes. Try and stick to a running plan to help you progress every week you exercise. To take it further you can even join a race or a charity run that you may want to support. Whatever you do make sure you enjoy your treadmill time then you will stick at it for longer.