How do I have an object created every 3 to 7 seconds?

Hello

I want to have an object created somewhere between 3 and 7 seconds. After the object is made I would like to have the cycle of a new object being made somewhere between 3 and 7 seconds.

I am thinking I use the "Every X seconds" under the system object but what would I put in the Interval amount?

Thank you in advance for any assistance.