This is a question I get asked a lot. It is a good idea to do interval training at least twice a week and three times is better. Interval training will help with your overall endurance so if you are training for a long running race, you can cut your running time in half according to a recent study.
A good place to start is with 30 seconds all out on a bike or running or any exercise. Then take up to 2 minutes rest at first. Repeat this at least 4 times and work up to 8 times. Eventually you will be able to cut your rest time way down to even 30 seconds. Your will be amazed at how long you can work out at a steady pace after training this way.
It is a good idea to check your heart rate after each interval bout to make sure it is increasing. By the last bout, you should be at your heart rate max or you didn't work hard enough. Happy training!!