"How could do this?"
Anywho, I wouldn't touch the "every X milliseconds" condition with a ten foot long clown pole.
Give the object an alterable value, call it "shoot" (what else?) then...
Object: Add random(3)+(2) to object.value('shoot')
Object.value('shoot') is equal to or greater than 100:
System: create object- bullet at object.x and object.y.
Object : set value "shoot" to 0
Modify the numbers here depending on how often you wish that object to shoot.
And this is all assuming you want an object to shoot something and that you know what to do from here (bullet movement or behavior).