It's a common question - the answer depends on your goals for the exercise you are doing. Are you trying to burn fat? Increase your cardio capacity? Intervals? I'll address those in a future entry, but I thought it would be good to have some charts for reference that you can kind of glance at quickly.
Here are a few for reference I found:
This one has suggested activities with specific age ranges
Here's a popular one for 10 second counts and ages: