My little niche at work is running "Tabata" classes. What is that exactly? In short, it's an interval training method that is based on timing :20 seconds of high intensity exercised followed by a :10 seconds rest period. And repeat! In long...take a look at this. It'll explain it in more detail. And better than I ever could.
Now, Tabata was made to only be done in 4 minutes. It will give you all of the wonderful benefits of exercise in that short amount of time. That's right...a FOUR minute workout. That sounds wonderful, but you really need to be pushing yourself to the max...as in 10 out of 10. The people I do classes with are not elite athletes who do this. They work hard, but won't break their back to give it their all. So...I make the class longer. It lasts an hour...including warming up and cooling down. Although, it's a full hour, it goes by super fast. And, if you are really going hard...you can probably just do 30 minutes of class. I'll share a lot of these workouts with you.
To make it easy, if you're working out alone, there are apps that you can download on your phone or iPad. The one that I use is Interval Timer by Runloop and it lets you set the amount of rounds/sets/music/etc and you can use it if you're doing other types of interval training like :50 seconds of exercise followed by a :15 second rest. Or just search tabata or interval timer and I'm sure you will find one that you will like.